If you're going to buy an aerobatic book, I would strongly recommend Alan Cassidy's "Better Aerobatics". IMO, it's by far the best book out there in or out of print (it's in print). Really an amazing effort and worth every penny. It's helped me a lot more than any of the others. I've read several others by Neil Williams, Eric Muller, Bill Thomas, and Szurovy/Goulian, and this one has the most detailed and in-depth descriptions and illustrations of the maneuvers (basic to unlimited level) as well as general flight dynamics. It's geared toward precision aerobatics, but that doesn't mean you have to be into competition to benefit from it.
The Williams, Muller, and Thomas books are good in their own way and each have a very different style, but if you're looking to gain the most understanding of the figures, the dynamics involved, and exactly how they should be flown, Cassidy's book will do this better than any of the others. If you're interested in a historical perspective on aerobatics, Neil Williams' book is interesting, but it's rather long-winded and fairly short on actual technique and detail. Bill Thomas' books are the easiest and most straightforward to read...completely no nonsense, but lacking a lot of detail. Eric Muller's book is good. IMO, the Szurovy/Goulian books are pretty lite by comparison. 'Better Aerobatics' is mostly the one I'll continue to refer back to.