Well...I'm only 1900 USCF, but I saw a big improvement in my play by studying the rook and pawn endings in a book by Max Euwe...I forget the title, but he also gives some great summaries on how to play other endings. For example I remember for Q+p[+p]+K vs. Q+K endings, best defensive setup is if your king is on the edge away from where your opponents pawns are, and your queen is centralized. Your opponent's is in a worse position if his Queen is on the edge. Obviously the Queen in the middle is good because it controls a lot of squares. But having your king on the edge is an advantage because it is the place where you are least likely to run into problems with your opponent blocking a check and giving check at the same time (pulling queens off). And if your opponent's queen  is on the edge, he is less likely to be able to block a check at all (which would help him to centralize the queen and have better chances of blocking a check with a check, etc.). So in that situation, the defenders perpetual chances are maximized. Anyway, I remember that without ever studying the actual positions, I was able to aim into this in a real game, and draw it. Also there is an explanation of the winning technique for bishops of same color when you are a pawn up, that was so good that I never felt it necessary to study the actual positions. Lots of stuff along those lines in the book. I remember after reading some of that book, I found myself always asking myself, "Which ending is favorable? Which pieces should I trade? Which should I keep?" So endings is my recommendation. Now what's your recommendation to me to get to 2200?!

Well depending on your opening selection it might be advantages of you to get teh CDs or DVDs on your openings. 

Books to increase general strength are MY great preddecessors by kasparov the vol.4 and 3 as the most important to help the initiative seeking folks. 

Dovertsky endgame manual is priceless and available on CD now so its not as boring just going over positions with a board. 

For middle game Euwes work is amazing and has withstood the test of time. 

Oh and i can only recomend partially the Gambit Cartel and some of the gambits he covers may even do well in the 2200and up area. You can download them from chess cafe .com