Three options:
1. Read the source
2. Generate doxygen documentation and then read the source, when something is not documented or is not clear enough in the generated document.
3. http://wiki.codeblocks.org/index.php?title=Developer_documentation
Not to forget, the most important:
http://developer.berlios.de/project/showfiles.php?group_id=5358
-> see under "SDK documentation (SVN)" the CHM files already compiled for you.
Thanks for your advice. I have a small problem yet, what's the difference between SDK documentation (SVN) and SDK documentation (release) ?
SVN is an acronym for subversion. Subversion is a tool to manage version of sources. It allow to track difference between them, and because of that, that kind of tools (VCS : Versionning Control System, CVS and SVN are the easier to learn. After them, you will find git, mercurial and some other, which do not use a server/client architecture and are harder to understand and use IMHO ) is always used (at least in open source projects) to contain the last version of source.
So, SVN means the last. Release are made from SVN, but only after testing. That last point differs depending on the development team: you could find releases which are frozen since a long time, and are really, really stable, but have many less features than the trunk (aka: main development branch), or you can find other softwares which use the "release early, release often" philosophy.