I own or have read most of the available books on vacuum tube amplification. While they all contain some useful information, there are not many I would consider indispensible. This book falls into that category. This book does not focus on the tonal differences between mustard vs. orange drops, or NOS vs. new production tubes. Nor does it offer descriptions of the features of classic amp circuits. There are plenty of books that do this already.
What this book does it to take you through the design of a circuit from beginning to end. Other books may tell you that the 5F6 Bassman uses a "long tailed phase inverter" and show you a schematic or layout. Merlin shows you how all the major phase inverters work, then takes you through the design of each type of phase inverter. All the calculations used for part selection are laid out in a straight forward manner. The pros and cons of each inverter type is explained. There are no unsubstantiated blanket opinions here.
The formulas may put some people off, but unfortunately, they are necessary if you want to modify or design amp circuits. This book does a far better job of presenting technical material than the Radiotron Designer's Handbook. In my opinion, the problem with the RDH is that it is not written for guitarists. In fact, the electric guitar was not even invented when my the version of the RDH that i own was printed! In this book, after going through the operational aspects and design of a circuit type, Mr. Blencowe will then often show an example of how the circuit is implemented in a well known amplifier. To me, the context is an important link that enables me to associate a circuit with something I have heard before.
The format of the book is similar to the material at Merlin's valvewizard website. There are no glossy color photos of amplifer gutshots, nor are there reams of schematics and layouts for popular amplifiers. There are many graphics in the book, mostly falling into 3 categories.
1. Circuit diagrams showing the components, voltages, etc.
2. Oscilloscope traces showing how a particular vacuum tube responds to a change in load or screen resistor, or how different distortion types manifest themselves in the signal waveform.
3. Sample layouts for circuits
Finally, this is of of the few amp books that I have read that I have used to implement design concepts. Specifically, I am "bootstrapping" the cathode follower and using "diode biasing" for the first preamp stage in the amp I am currently building. Both of these ideas were lifted right out of this book. If you are interested in getting beyond the "paint by numbers" method of building amps and want to design your own amplifier, or modification to an existing amp this book is on the short list of amplifer books that is indispensible.
As yet, I have not bought the companion book on power supplies. It is definitely on my list, however.