I have all of those books except the last one on powerscaling.
I have been an electronics engineer for nearly 30 years now, but I have never come across better books on contemporary circuit design and application, even in semiconductors.
The books are expensive, but if you get TUT 2 (the book it started with), and maybe TUT (1) on power supplies, you are very well off.
I enjoy those books and used to have a very good contact with the author, however at some point I must have said something wrong and he grew downright insulting. To the day it is not quite clear, what that was.
The author describes how basically anything can be done in the signal domain (preamp), yet continues with another series of expensive tomes on modding of power amps to achieve the same tones. I have pointed out that he was self-contradicting and ambiguous on that and maybe that was what he did not want to hear, but this is only assumption. I was pretty concerned about the whole thing and was trying to resolve that, but it seems I failed. Despite my approaches, I never received the truth on that.
Basically the series of books describes, how you hot rod your tube amp into a fancy overdrive machine.
I employed some of those ideas and modified a tube amp, but, besides its stock operation, you are limited to just so many sounds. I was not overwhelmed with many of them overdrive sounds.
As a result, I use the knowlegde I have gained and, these days, if I get an amp to repair, I rather resotre it to its initial beauty and simplicity. All other things can happen outside at a much easier scale (stomp boxes, digital effects processores, Kemper type amps...).
There will be a small niche of people that enjoy their hot rodded sound, and just that, so this is the right way for them. For me, versatility is the way, and with all those bells and whistles, you rather reduce versatility IMHO. The books also describes many of the design flaws that have come about due to marketing reasons or just plain penny pinching and how to overcome them. They also describe how to avoid to fall for marketing arguments by contemporary tube sellers.
I suggest you get TUT2, which is not too expensive, and outperforms any of the tube amp books on the market. With this knowledge you are able to restore your amp faithfully to its original condition. He approaches his subjects (amps, speakers, musical instruments) on an engineering basis rather than a sales basis. However, towards the end, I get the impression, he gets entangled in his own sales trip web, both books and kits.