Gerald Weber is brilliant! I can't say enough about his talent. This book, however, is not a teaching book per se. It's a book that includes his insights from years of doing mods. Its like sitting down with him and interviewing him. Its not didactic, it's more like sitting at the feet of a wise teacher, listening to him ramble!
For example, he discusses biasing, but he refers to earlier books/DVDs for exactly *how* to do the biasing. He talks about mods for all kinds of amps, but when you have questions on the "how to" you have to look elsewhere. So again, it's a fantastic book for seeing the big picture of amp mods, full of insights and tips, but it's not a bonafide "how to" book, or a repair manual.
In other words, I think this book is for players who want to understand better what the issues are in doing a mod and creating better sound, while reading though insights concerning lots of different amp-types. But if you're looking for a book to explain how to do a bias, how to do a certain repair, this doesn't cover all the necessary details.