I'm a casual player who's trying to get better at backgammon. I've worked through 501 Essential Backgammon Problems a couple times and played/reviewed a bunch of games on Snowie. What are the next few books I should pick up? Any other good resources I should know about?

Walter Trice - Boot Camp, in my opinion the best book I ever read.

Here are a few recommendations:

General Theory:

Backgammon (Magriel)
Winning with the Doubling Cube (Bell)
Modern Backgammon (Yours truly)

Specifically cash game problems:

New Ideas in BG (Woolsey)
BG Encyclopedia (Woolsey)

Specifically tournaments:

How to Play Tournament BG (Woolsey)
Marmorstein versus Greiner (Woolsey)

Tom Keith's web site Backgammon Galore has an excellent section on books:

http://www.bkgm.com/books/index.html

On the same site, you should check out the many excellent articles he has collected.

My next purchase is going to be Robertie's book MODERN BACKGAMMON.

The http://www.thegammonpress.com
has a terrific selection of books. They also offer articles, equipment, out of print material, lessons etc.

I have had great experiences with thegammonpress.com

It is Bill Robertie's site.

Magriel: Backgammon

Do not trust the advice about "action plays" and according to the modern bots flexibility is overvalued. There is a German web site(don't remember the name) with analysis of some of the plays where the bots disagree today. But overall a fantastic book. Pay attention to the "Safe versus Bold Play" section.

Trice: Backgammon Boot Camp

A very good book that also advanced players can learn something from. Covers wide range of topics. It has a bit of a disjointed feel to it due to the way it was created(Small individual articles). Very few typos and mistakes.
A bit short on explanation at times and none of that technical stuff utilising math that Trice was famous for. This is probably the new modern Bible after Magriel.

Robertie: Modern Backgammon
The first part of this book is simply the best book on checker play there is. Period. Second part is a complete match annotated with often deep analysis and contains enough to make it worthy of having been a book on its own. I have read the first part several times, and every time I do so, my checkerplay errorrate improves if I have become a bit rusty. Well written and I have found only 2 mistakes in the first part which I have studied carefully. This book is extremely good value for money. Rereading Modern BG always makes me hope that Bill will leave that Poker thing and go back and write a new BG book !

Woolsey: New Ideas In Backgammon
Good book with some very challenging positions on average. Be careful to read it at the right time as it contains mainly situations where an unusual play turns out to be right according to Jellyfish rollouts. The analysis is good but it does show, that this is a make-the-analysis-fit-the-rollout book. However there are great descriptions on how to handle and what to think about in many difficult but often occuring situations.

Bagai: Classic Backgammon Revisited
I Havent read this one but it has gotten great reviews since it came out and the sample chapters available look to have some very good analysis.

A few comments on other books mentioned:

Vernon Bell's book on the doubling cube. Contains a lot of bad advice, so much that I would recommend not reading it.

Woolsey on Tournament Backgammon: Highly overrated. If you have access to a match equity table and is a thinking person, there is close to nothing in this book.

Good stuff, thanks a lot guys.

Backgammon boot camp, Boards, Blots and Double Shots, and 501 problems are my 3 favorites