I'd been looking at amp books for a while but never found one I liked. I was primarily interested in understanding an amp, any amp. I was not interested in history of companies or being offered easy mods without explanation what's going on, e.g. "change the feedback resistor" without saying what the feedback circuit does, etc.
The present book gives an overview of the most common components, explains tube operation in general, and explains layout of amps to some extent. It is basically what I needed: a concise description in easy reading format. I finally have a rough grasp of tube amplification, what all this class A and B stuff means, what biasing is, where distortion comes from, how tremolo and reverb work, and what the stages in an amp are.
People who don't know how to read circuit diagrams or don't know the basics of resistors, capacitors, coils, magnetism, and fields might find the book frustrating. The book does not go into too much depth in some instances. For my purpose that's okay. The book is cheap and short, gave me the big picture, and I know now what I'd like to understand in more depth and what I need to look for next.

This book is informative and jumps right into technical explanations of tube circuitry, and includes a handy appendix in the back for internal vacuum tube construction and pinout information. It does have useful information in it. Just a note: although the Amazon description says 80 pages, it is really only 60 pages. I was surprised when I received it because it is constructed very cheaply - it was just papers stapled together in two places. I have effects manuals better put together. I'll probably have to make copies for backup because I don't see this lasting too long on the workbench or bookshelf without becoming torn or overly worn. If you can live with this then by all means, purchase the book, because it has some great info for the vacuum tube amp tech/enthusiast.