When I started a programming job, I read this book, Clean Code, and Code Complete. Code Complete is kinda old but still great, Clean Code is not bad but it's Java centric and has a lot of questionable tips. But The Pragmatic Programmer never gets old.
I got on very well with Van der Linden's Deep C Secrets. It's from 1994, so misses out on the newer versions, but aside from that it's aged well, IMHO.
Are you sure it's been open sourced? I'm reasonably sure you've linked to a site offering pirated copies.
There are several links to PDF versions of the book. None of them include either a copyright page or a statement that it's been released as open source.
The author's own website <https://afu.com/> includes errata for the book, but doesn't provide or mention a free copy.
A free sample of the Kindle version of the book does include a copyright notice. A book published in 1994 is not public domain unless it's been explicitly released.
Something that appears to be a legitimate PDF sample (not the while book) is here:
