While not about specific engines, I am quite sure that there is enough literature about "how to build your own engine", as I have enough of them on my bookshelf since MS-DOS days.

These were my first two books on the subject:

https://www.amazon.com/Tricks-Game-Programming-Gurus-Andre-L...

https://www.amazon.co.uk/Building-3D-Game-Engine-C/dp/047112...

So I imagine a local library or bookstore might have some dusty books from Andre Lamothe, Game Development Gems book series, and whatever else came after.

> One is about doing a return to castle of wolfstein clone

Is it "Tricks of the Game-Programming Gurus" (https://www.amazon.com/Tricks-Game-Programming-Gurus-Andre-L...)?

> a 3D engine in C++ for a space shooter

The other seems to be "Building a 3D Game Engine in C++" (https://www.amazon.co.uk/Building-3D-Game-Engine-C/dp/047112...).

It's amazing to see old gamedev books :)