The Complete Guitar Reference
In this e-book you will find a whole set of different tutorials that show you how to get the most out of the chord diagrams, song writing charts, modulations and capo references. If you don't understand what I am talking about, this book will help you to do so. Personally, I think, that every guitarist should have read The Complete Guitar Reference, because it has all the information you need to start creating your own songs and become a better guitar player.