[ssrc]

I remember the books on undocumented functionality like

https://archive.org/details/Undocumented_DOS

[cout]

I remember that book! I wanted to get a copy but made do with the DOS Programmer's Reference, 3rd ed., which was also very good since beej's interrupt list wasn't available yet or I didn't know about it.

[rzzzt]

Was it Ralf Brown? beej has C and networking tutorials: https://beej.us/guide/

[cout]

Yes, Ralf Brown. I guess I'm getting old.

[rzzzt]

I found a copy of the PC Game Programmer's Encyclopedia (PCGPE) back then but couldn't really wrap my head around most of the articles (especially the 3D stuff and Mode X).