In this chapter, we were able to only give you a very basic introduction to what is possible in C. Here is some advice on how to get more information.
First, there is of course the chapter C-Language Functions in the PostgreSQL manual. This can be found online at http://www.postgresql.org/docs/current/static/xfunc-c.html and as with most of the online PostgreSQL manual, you usually can get to older versions if they exist.
The next one, not surprisingly, is the PostgreSQL source code itself. However, you will usually not get very far by just opening the files or using
grep to find what you need. If you are good with using
ctags (http://en.wikipedia.org/wiki/Ctags) or other similar tool, it is definitely recommended.