If you are familiar with Wendler's other book 531, and like it, then this is an absolute requirement to purchase. I have read through my copy of Beyond 531 about 4 times now, taking notes, and it contains tons of new info and new variations to training. It's almost like Wendler just said screw it and poured all his training knowledge into this one book. Essentially what you get are unlimited new spin-offs to the original 531 program. For example if you want to use more volume, there is a plan. More singles, there is a plan. Higher frequency, there is a plan. Peaking for a meet or increasing strength, there is a plan. He even includes a program in the works called Beyond 531, which is very interesting. Particularly I found the most useful bits to be what he refers to as the Last Set First, Joker Sets, and Spinal Tap Training to be the best parts of the new book. He also lays out a 28 week training block where you manipulate the classic Boring But Big plan from 10 rep sets all the way down to singles.
So if you haven't purchased 531 yet, I would say start there. This book is a completely new addition to his methods and doesn't really explain the original 531 book, since his first book did. Rather this book provides tons of new options and ways to modify one's own program to fit their needs/schedule/training goals. The best way I can explain it, is it's like if your favorite band released 100 songs all at once, instead of narrowing it down to a 12 song cd. He puts it all out there and lets you decide what is useful and what is not for your training. What's funny is that ever since about 2 years ago when I started following the 531 plan, people have asking me what I do in the gym or what complex lifting program I follow! I have even been accused by friends, of using steroids because of the mass/strength you can build with this plan. Little do they know about 80% of my work is just different rep/set variations of the squat, bench, deadlift and military press all thanks to Wendler! Highly recommended.