I think the list will complete with books on networking protocols.
TCP/IP Illustrated, Volume 1 by W. Richard Stevens I found this to be one of the best books to understand how Internet protocols work. I have read the first edition, have not read the second one.
Companion book on Unix socket programming, I like the first edition which is very concise, but expects a lot of Unix background.
Unix Network Programming, First Edition by W. Richard Stevens
Or subsequent edition, if you want more explanation, equally good.
Unix Network Programming, Volume 1