I swear by "The Complete Reference" series of books, I have the fourth edition C one (Schildt) which I still refer to even now.
Otherwise I'd push you towards specified books - i.e. advanced GTK manuals and the like.
Networking tends to be different depending on your OS, but I recommend UNIX Network Programming (Vol 1 & 2) for Sockets and IPC. Coupled with "beej's guide to network programming", I only need to reference other documents when I'm working with OpenSSL or more advanced IKE interfaces.
Other than that, see what everyone else recommends!