Wher can I find a good documentation on these two PHP files and a more detail & indepth use of each function?
Facebook documentation is... not informatitve eno
I feel your pain.
facebookapi_php5 is mainly each API method. facebook is mainly for session/auth.
The best place I've found so far is the developer forums, but it is mainly questions without answers.
Since Facebook is always changing their API, the documentation is inevitably lacking and/or out of date. If you are looking up viral information, you going to find a lot of pages that say they have been deprecated.
My most used pages are: http://wiki.developers.facebook.com/index.php/API http://wiki.developers.facebook.com/index.php/FBML http://wiki.developers.facebook.com/index.php/FBJS
You can play around with the API here: http://developers.facebook.com/tools.php?api
But in the end, just about everything in those files gets passed through the post_request function in the facebook_php5_restlib file. That's where the curl function is for "posting" to Facebook. You can always capture what's going on in that function.