Thank you for your reply!
I did a search for the book you mentioned and found this link which leads to a Microsoft article on it. I mention this because that article points to this github repository of supplemental material (MIT licensed) that might be of interest (but not obvious).

The recent source code leak has brought to mind an old question I've had for some time. Namely, would it be possible or practical to use ReactOS as a way of studying windows internals -or has windows diverged too much since 2003 to get much of an idea of how things work today? How would one go about...

IoI_xD wrote: ↑Fri Apr 03, 2020 7:27 pm I'm sure the developers are having fun making this operating system, and I'm sure the community is having a lot of fun with it.

Congratulations.
You just answered your own question.