What are some resources for someone past beginner but not yet intermediate? I've been playing chess off and on for the better part of 15 years, however, I've never done it enough to count myself as any good at it. I'd like to get better, but intermediate guides go over my head, and "chess for beginners" is all about rules and simple stuff I picked up a long time ago. I'm looking for something that can help me improve from my weird in-between state to an intermediate level.
I will go ahead and assume that your playing strength is roughly at ~1100 Elo, since that is what I consider to be the approximate level of an "advanced beginner", so to speak.

To a player of your level, I would give the following advice:

Chess is a game which consists of three main phases: The opening, the middle game, and the endgame. These three phases are characterized below.


The opening: The part of the game where both sides have yet to get their pawns and pieces into action. Both sides should strive to take control of the center, develop their pieces, and put their king somewhere safe.
The middle game: Both sides come into attack range of each other, and a battle for the advantage is begun. Many things happen, and the ability to think of "a plan" is vital not to get lost completely.
The endgame: If the game is not already decided in the middlegame, one can expect that both players are somewhat comparable in strength, causing them to eventually trade down pieces until not very many are left. Since there are not many pieces left, the ones that are still present matter more than ever, and tactical ability combined with concrete knowledge of basic endgames is key to playing the endgame with success.


Q: What does an advanced beginner need to know about the opening to up their game? 

A: In the opening, one needs to know about the basic opening principles, and one should strive to apply them into their games. For instance, look into the thread Good openings for a beginner (for white), and xaisoft's answer, which outlines some opening principles. The following link also gives some advice on how to conduct one's opening play as a beginner: https://www.chess.com/blog/NimzoRoy/chess-opening-principles.

Q: Why is it good to follow the opening principles? Why shouldn't I just learn concrete opening variations?

A: The opening principles are the easiest means to get you to a playable middle game, and if you follow them you would be surprised at how often they will succeed in guiding you towards the right moves in the opening. As to why one should not focus on learning concrete opening variations as a beginner, it mainly has to do with the fact that it's too early for a beginner to do that. 

The reason you study specific opening theory is to try to get a concrete advantage out of the opening, and at the beginner scene, most players don't know opening theory, and will thus not very likely play the exact lines you've studied. This means that you will have to spend an unreasonably big part of your studying on openings, which will impede your progress in the other two major phases of the game. Also, as a beginner, it is extremely difficult to learn specific openings well, since that requires understanding of the ideas in certain positions(which is very hard to attain if you don't know much about planning in chess in general), and not just knowledge of precise variations.

Q: Ok, so I applied the opening principles, and got my pieces out and all that. What's next? What about the middle game?

A: Following the opening principles, unless you're facing some really nasty opponent, the following will most likely happen:


You will most likely have a pawn or to in the center of the board.
Most of your light pieces(knights and bishops) are likely out of their starting positions.
Your king is likely castled already.


Now it's time to start looking for short-term and long-term goals for you(and for your opponent!). In the middle game, the thing that will make you reign supreme over beginners is tactics, and plan-oriented play. Most beginners are not very procifient at tactics, and do not play their moves according to a clear-cut plan. 

Q: Ok, so what are tactics? How do I learn them?

A: The term tactics is an umbrella term that involves concrete, often forced, sequences of moves in a position. These sequences can be one move long(like capturing hanging pieces), or more long, and complex(like conducting a well planned attack successfully against the opposing king).

Normally, the difficulty of a tactical sequence is determined by how long it is; shorter sequences are easier, and longer sequences are harder.

As a beginner, one should focus on learning to find these shorter tactical sequences(1-3 moves deep) consistently. To do this, use the following guidelines for calculating(looking for) tactical variations:

THE GOLDEN RULE FOR CALCULATING TACTICS

At every move in a tactical sequence, consider the responses in the following order of priority:


First, consider all the checks in a position;
Then, consider all the possible captures in the position;
Consider other moves.


Using this will help you to spot the details that make or break a tactic. 

But starting out with tactics, one should definitely look for some common tactical ideas that will further aid one's play. Here is a good list, with examples, of a wide array of tactical ideas: https://www.chess.com/article/view/chess-tactics--definitions-and-examples. Learning how to apply these in one's games is not easy, and practice is crucial. Playing games is very good to improve calculation, and solving tactical exercises, on for example http://chesstempo.com/, can be very beneficial as well.

Q: Ok, great! So, what about plan-oriented play? What is it, and why is it good to learn?

A: By plan-oriented play, I simply mean that one should always make a move according to some plan, and not just make an arbitrary move with no clear purpose other than wanting the game to progress. Bent Larsen, one of the great players in the 1960's and 1970's, has the following very famous quote attached to his name: A bad plan is better than no plan!

What this means is that a you need to have a goal in mind - however misguided - to ever hope to beat your opponent. If chess were boxing, this advice could be that no matter how unrefined, it's better to throw punches aimed at your opponent, than it is to just throw your arms around aimlessly and run around in circles.

As a beginner, go for simple plans, like trying to win a pawn or piece by threatening it with your pieces. Or you can try to use tactics, to gain material or to try and mate your opponent's king.

The plan can even be long-term, like "I don't want to get checkmated on the first rank, and I don't want my knight on f3 to be pinned. Therefore I will go h3, to create luft for my king, and prevent my opponent from stepping on g4."

If you don't find an immediate plan, it could help to "talk with your pieces", and try to improve their placement. By this, I mean that you should try to evaluate if a piece is badly placed or not, by trying to give your pieces some personality traits:


The knights like to stand in the center, where they can move very freely, and control the enem forces. Even better if they are defended by a pawn, and there are no annoying enemy pawns to chase them away! The knights don't like very much to stand on the edge of the board, which leaves them restricted. But as long as it's not a corner square, it may be tolerable. In general, the knights are stronger in closed positions(not many open lines).
The bishops like best to stand placed on long, unrestricted diagonals. This way, they can be relatively safe, and at the same time work to make the opponent's pieces stumble over each other to avoid the scary bishops in the distance. Since the bishops are so mobile, they like to have multiple targets in the enemy camp. The bishops dislike being treated like pawns, and they absolutely hate having to be trapped behind their own pieces, with no hope of coming out to play in the near future! The bishops are stronger in open positions(many open lines to work with), with pawns on both sides of the board.
The rooks like open ranks and files, and in particular, they love to work together. The best thing they know is to "double up" on an open file, and then just crash into the enemy's position. This can be devastating for the opponent, since the rooks are real powerhouses. In particular, the rook likes to go on the seventh rank, where it will run around, scaring the enemy pawns, and severely restricting the movement of enemy pieces. Rooks are, like bishops, stronger in open positions, with open lines to work on. However, they are also more vulnerable, since they are more valuable than a bishop or a knight.
The queen boasts with unparallelled power at the board. She likes to spread terror in the enemy camps, and will gladly cooperate with other friendly pieces to get her way. She likes the center, where she has free access to almost any part of the board, but she hates getting in the line of fire from the enemy lines(that's no way to treat a lady!), and wants to stay safe, if possible. Therefore, she often waits until after the opening to get out, when the coast is clear, in some sense.
The king is the most important piece, and he knows it. Therefore, he makes a point of staying safe at all cost. During the middle game, when the battle is in full swing, the king wishes to sit back and ride out the storm in safety, behind his loyal pawns and pieces. However, when the dust settles somewhat, and the endgame approaches, he feels it's a lot safer to go out and show who is the real boss! Boasting with striking power close to that of a rook, he will join up with his allies to end the battle, using his flexibility to drive away those who dare oppose him. However, he is still quite gun-shy, to be honest.


Knowing your pieces is the fundamentals to learning how to use them properly, and it's a vital part of learning how to go from beginner to intermediate player(it's at the core of so-called positional play). Searching for "planning in chess" should bring you some helpful online resurces.

Q: Do I really need to study endgames if I want to improve? They are boring, and I never even reach them in my games, so what's the point?

A: If you really wish to improve, the endgame is essential to study in some detail. However, the endgame has an unfair reputation of being boring. Granted, there aren't as many "flashy" moves. However the tactics are never-ending, and in many cases the endgame is a battle on the knife's edge. So even if it doesn't look very flashy from an onlooker's perspective, I can promise you: it is when you learn to truly appreciate its many finesses!

Also, if you're never reaching endgames in your own games, it often means that either you or your opponent tries to avoid the endgame at the cost of a potential loss, or it means that your games are very lop-sided(someone gets mated or loses many pieces early on). As soon as you play someone of equal ability, you won't get fooled by your opponent's tricks, and your opponent will not very easily fall for your traps either. Therefore, to win, you will need to be able to outlast your opponents, and be prepared to face them in the endgame. Expecting never to reach an endgame is equivalent to expecting that at least one player will make a severe mistake early on.

With that being said, at the beginner level, you don't really need to learn everything about endgames immediately! Learn the basic mates with rook, queen, etc., and learn the basic concepts of pawn endgames(there's a wiki page on this topic here: https://en.wikipedia.org/wiki/King_and_pawn_versus_king_endgame). Some basic concepts of rook endings could also prove useful, and in general, the more endgames you study, the better! However, don't push yourself if you can't stand studying endgames further. If you want a good book on endgames to use as a beginner, I'd recommend Silman's Complete Endgame Course. It divides the different endgames after player ability, rather than piece configuration, and you can use it to improve later as well. 

That concludes it, I think. I cannot think of anything more I'd say to an advanced beginner seeking to improve. Good luck with your ambition!