I have acquired quite a few BJJ books. I can't help it. Whenever a new one comes out, I have to buy it. And Santa was most obliging this year with the provision of two new titles for me to pour over Christmas.
But if I asked myself, how useful are these books actually? I would say that they only offer me a small return on their investment. Mainly because you cannot substitute good hard training on the mats. But also because I am a lazy reader and don't really study them properly, I just browse through random pages when I am on the bog and make a mental note of moves that interest me. If I remember, I may try some of them out in class, but mostly I don't. What they do help with though, is if you have just recently learned a technique and need some pointers for fine tuning, or to reveal variations on the technique. They also reveal stuff you may never have tried before, so you can at least be aware of them. So with that in mind, here are my top recommended BJJ books so far:

1. The winner of the 'I do traditional martial arts and would like to see what all this Gracie stuff is all about' category:
Theory & Technique, Renzo and Royler Gracie - this book was criticised when it first came out for its random assignment of techniques to belt levels. Despite its flaws, it has stood the test of time and is a good introduction to the art. A review can be read here.
Honourable mention: Brazilian Jiu-Jitsu Basics, Gene Simco - His 'Master Text' is too weighty to be of practical use, but Simco's beginner's guide is not bad. Though the b/w pictures are a bit poor in some areas.

2. Winner of the 'I have been doing BJJ for a few years and need to learn some sh#t hot moves now' category:
X-Guard, Marcelo Garcia - this book is so beautiful it could only be authored by one man. Marcelo Garcia is such a legend I really don't think I am worthy to even try these moves out. A good review can be seen here.
Honourable mentions include:
Advanced Jiu-Jitsu Techniques, Fabio Gurgel (small handy size, great ready to use techniques, but not great pictures).
Black belt techniques, Jean-Jacques Machado - Cool fancy moves that the Machados are famous for.

3. Winner of the 'Oh no my instructor called in sick and said I have to take the class, what now? category:
Guerrilla Jiu-jitsu, Dave Camarillo, Erich Krauss - wanna learn to fly? this book is your flight manual and first class lounge all rolled into one neat package. A whole chapter dedicated to beating judo guys and another on flying submissions...awesome! A good review here:
Honorable mentions should also go to:
Jiu-Jitsu unleashed, Eddie Bravo, what can be said about the man Bravo that has not been said before? Whatever your view, this book introduces his unique take on BJJ. Not very good pictures, but subsequent books offer more detail or free on youtube, a zillion guys trying their rubber guard or Zombie or whatever he calls it. Actually I quite like his stuff, but there you go what do I know?

4. Finally, 'most awesome BJJ book ever award'
Jiu-Jitsu University, Saulo Ribeiro - I only just got this book but already it looks so amazing that I can't take my hands off it. The sheer variety of techniques on offer and more importantly, the philosophical points raised by Saulo make this book stand out. Ribeiro is a brilliant instructor, an elite competitor and now, an author of probably the most definitive BJJ book of them all. Well, until Marcelo publishes his next book.
Full review here.