For the base language (as you'll see repeatedly), "The C Programming Language". For the standard library (POSIX), I don't have a good catch all source but you'

For the base language (as you'll see repeatedly), "The C Programming Language". For the standard library (POSIX), I don't have a good catch all source but you'll want one here. For networking (which tells more than just networking), "Beej's Guide to Networking". That's a fantastic book (online free from the author). Hilarious and informative. I liked writing existing unix utilities myself and I found that a helpful set of projects.