Beej’s Guide to Network Programming Using Internet Sockets

Author Hall, Brian
Find it WorldCat
Project Gutenberg
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Originally an internet tutorial, now available in print via Lulu, Beej’s Guide is clear, concise, easy to read, easy to follow, and all around great. Obviously the scope is very narrow (it teaches you BSD/POSIX socket programming in C, der), but if that’s what you need to learn or brush up on, I can’t think of a better source.