I recomend Alan Cassidys book Better Aerobatics
Better Aerobatics: v.1: Vol 1: Amazon.co.uk: Alan Cassidy: Books
or
Better Aerobatics Book by Alan Cassidy
Its very good on the basics but has a lot of depth if you want to go past the basics.
I found Duane Coles book only covered the basics.
Neil Williams book has a lot of interesting info other than how to fly aerobatics so is an enjoyable read.
Alans book is really only about aerobatics so is the best of the 3 in my opinion.
ZA