Failing to understand a pin from a tactics book I've began going through, over the board, 1001 Winning Chess Sacrifices and Combinations - 21st century edition. This book is a great supplement to my last question on studying tactics, and I feel like I'm learning a lot from it.

One of the problems I'm having with this particular book is right out of the gate the tactics for pins are very difficult to see in a lot of cases for me.  Here is an example, problem 14:

[fen "r4rk1/p2nqppp/Qpp5/3p4/3Pn3/2N1PN2/PP3PPP/2R2RK1 w Qq - 0 1"]


To me one move was clear - 


Nxd5


This move clears the way for the rook to enter and pin the knight to the queen. It's almost forced to take with the pawn to prevent the queen capture, the queen is on a pretty good square all things considered. Initially I had evaluated:


Qb7


But disregarded it as a first move because even though the knight is pinned to the queen


... Rfb8 


Mitigates the attack immediately and forces the queen back - or so I thought. By having the rook perform the pin I'd be in better shape to mitigate that attack when I do 2. Qb7.

After:


... Rfb8
Nxd5 Rxf7
Nxe7+


Which seemed too easy to be right for this tactic and leaves the game at even money, which from what it seems is never the goal of a pin.

So from what I've gathered here I saw the pin at least. The problem I'm suffering from at the moment is how the solution plays out - the solutions don't really give great justification and there's a lot of moves here to consider it "solved":


Qb7! Rfc8
Nxd5! Qd6
Rxc6!!


From this I'm curious


Why 1. ...Rfc8 and not Rfb8, attacking the queen? 
Why does black resign after Rxc6!!?
How do I study from this tactic book effectively? I'm getting a ton of them wrong even though I'm seeing the tactic, I'm unable to make the same moves in the tactic for black to make everything "play out right". I'm not sure when to consider a solution "complete".


I'm sorry if this is trivial, I'm working on fixing my over the board play by doing tactics and this book is incredibly difficult. Difficulty is great because its challenging but I'm only on problem 14!
To me one move was clear -
  
  
  Nxd5
  


But after 1...cxd5 2.Rc7, although the knight is pinned for a move, Black can easily unpin it by moving the queen to d6, d8, e6, or e8.  In fact, the knight is not even fully pinned - Black could move it to c5 and force a queen trade.  You've sacrificed the knight for a pawn, and get no lasting pin.


  Why 1. ...Rfc8 and not Rfb8, attacking the queen?


Well, after 2.Qxc6 Black has lost a pawn.  And after that move the d5 pawn is threatened twice and not defended, so Black must play 2...Nxc3 if he wants to have a shot at saving it. But now he's traded off a well-placed knight, and after 3.Rxc3, d5 is isolated and still needs a defender, and White will gain solid control of the c-file and will be up a pawn.

Rfc8 is shown here, not necessarily because it's Black's best move, but because in order to figure out how to proceed against it (and that Qb7 is a good move in the first place), you need to see the pin that Nxd5 exploits.  (And if this is based off an actual game, it might be the move actually played in the game.) 


  Why does black resign after Rxc6!!?


Because he's down two pawns, and White will control the only open file.  There's a threat of Ne7+ which limits Black's options in the coming moves.