Detailed documentation for external calls (threading, shared resources etc)

Greetings,
This is actually a request for documentation guidance. I intend to
develop an extension to postgresql. Basically I'd like to place calls
to network using ZeroMQ, and I need to have detailed information about
a lot of things, especially  threading issues. I need to have some
global resources which will be presumably used by  multiple threads.
I can see that there is a lot of documentation, but I'd really
appreciate pointers towards the books, or key documents that'd help me
move forward faster (docs/books about inner workings of key
functionality) I'll be using C (most likely the best option) to
develop code, so which books/documents would you recommend?

Cheers
Seref