Could someone give an overview of what is covered and whether it is still worth to invest reading these books ? Are any of the techniques still valid ?
To answer your question would depend on what you want to gain from the book(s). Are you looking to solely learn how the games were made and work to learn their techniques to apply to your own projects? Are you wanting to understand the history and other things that surrounded the period of development, and why certain choices in coding were made? Are you just a fan of historical perspectives in PC game development?
I think for each of these questions, the answer would be that there is value in investing time reading the book(s), but that level of value will be different for each person and for why they are thinking of reading the books.
I think if you had no idea about how these games worked or had never implemented a similar engine yourself, that they wouldn't be the best source to go to in order to learn how to do it. There are many other sources, in books, on the internet, and in various archives that will have details to teach you far better.
However, after doing so, you may still have some questions relating specifically to W3D or Doom about how their engines worked - and that's where these books (based on what I know from reading the W3D book) shine - they give you a fairly well written account about how these engines work at their core. I suspect that they also may help you understand the techniques you've already read about and/or implemented better - or give you a different perspective on them.
As far as whether the techniques are still valid?
Well - if you're interested in building a pseudo-3D engine in software, and understanding how that works, how to optimize things, etc - then yeah, those techniques are still valid. They are probably good techniques to know which tricks might still be applicable even in today's hardware powered 3D realm - knowing how to do these tricks might inspire a solution for, say, a shader implementation to eek out a tad bit more performance or something? Or maybe you just want to understand how old-school games did what they did?
If you're asking whether such techniques could lead to creating the next breakthru AAA game - well, unlikely, unless you did something compelling and the idea of it being completely done in software and old-school was part of its selling point/strategy...
Even if you don't directly use any of the techniques,I think books like this give you a better appreciation of how to make your software more efficient and how well architecture software and choices in data structures and algorithms can massively improve your software. And more efficient software is something I think we should all strive for