Learning steps of a non-professional or play-for-fun player Let's say I already know the rules of the games. What would be the next thing I should focus on? And then? Learning the most common openings? Unbalances?

I learned to play chess as a kid, played once in a while back then but then I have hardly played in 20 years. I've started to play again, and I'd like to improve. I don't want to be extremely good and I don't have a lot of time to invest, I'd just like to achieve a decent level and enjoy playing.

I'd also appreciate a couple of book recommendations, pre-intermediate level.
Go to your local chess club. They will match you with opponents of an appropriate skill level. In addition, there will be people there willing to help you improve.