Is studying tactics THAT important?

If so, what are the best books for learning and improving at tactics? (I already utilize the tactics trainer on this site.)

There are a million books on tactics, i dont think it really matters what book.  I will say that Susan Polgar's and Lazlo Polgar's books are very good.

chesstempo.com is free and very good.

To answer your question...I think tactics are just as important as any other part of the game.  Tartakowers quote: "Chess is 99% tactics" has been debated until everyone is blue in the face.  Joel Benjamins quote: "Chess is 100% caluculation" best illustrates that all phases of the game need to be studied.

Personally...tactics is the part of the game that i put the lest effort into, and i have been able to make USCF A player. 

I've heard both as well. So I guess I need to learn it all. My follow-up question would concern what books are best for tactics and what are best for calculation (for various player ratings).

Tactics:

Susan Polgar's and Lazlo Polgar's books are very good

Calculation:

Chess Strategy: Move by Move - Adam Hunt

Susan polgar has a whole book on tactics.