Comer/Stevens' "Internetworking with TCP/IP" is a great book for TCP/IP sockets. "Unix Network Programming", by L. Richard Stevens, is another excellent book about sockets programming.
But I'm not sure either one is particularly helpful for "Posix"
, which is much more than just "sockets".
For Posix in general, any good book on "Unix" or "Linux" should give you the background you need.
The best reference, of course, is the collected Posix Standards
. But they'll cost you