[cb321]

Most well vetted material you will find on this will use regular old C, not Nim, like that Bryant & O'Halloran Computer Systems from a Programmer's Perspective mentioned in a sibling comment. It probably won't rely on C that dramatically, but your best bet today will probably be to learn the used subset of C and Nim analogues at the same time.

Your questions are a bit all over/vague. The Nim forum (forum.nim-lang.org) is generally very helpful and if you ask more specific questions then you can get better answers.

There is also https://ssalewski.de/nimprogramming.html which has some material on what you are asking about.