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 :)

I asked about this specifically because, when I got the book that really started me deep into programming [1], I was hampered by my lack of a C compiler and assembler, and the ~$200 cost thereof ($400 total, in mid-1990's dollars, for the two tools -- kids these days have it easy with binary downloads of Windows ports of gcc and NASM available free on the web). If I'd had that kind of money in middle school, I would have blown it on video games. (I didn't know about GNU, I don't think the DOS port of gcc, the Allegro system, was yet born, and I didn't have any mentors.)

The source is messy and compiler-specific, and many implementations are irrelevant to modern systems. For example, I recall Chapter 14 is about implementing multiplayer over serial cable by directly writing to I/O ports in assembly language to drive the UART. I never did get that to work, but trying to translate the book from the Watcom C + MASM toolchain to the QBASIC + DEBUG toolchain -- without access to a working copy of the former and zero prior C experience! -- actually taught me a lot.

So maybe there is a reason toolchains should be expensive -- to encourage starving students with lots of time but little money to expand their minds by jury-rigging alternatives :)

[1] http://www.amazon.com/Tricks-Game-Programming-Gurus-Andre-La...