I've been looking around the site for documentation on how to develop for XOOPS; more specifically, how to develop new modules and blocks - files to create, API to implement the XOOPS core will call, classes I should know about, how to integrate with the XOOPS administration system, etc., but I haven't found any guides (or tutorials). Where should I start?

Thanks in advance.

(Note: I forgot to say I have proper knowledge of PHP, HTML, CSS, SQL, and portal concepts. I just need to know what should I implement to develop modules and blocks and where can I find documentation of the XOOPS APIs beyond the headers available in Development: API Doc I can generate myself.)

The best documentation is XOOPS itselfs, than the developers forums on dev.xoops.org.

http://docs.xoops.org/
http://docs.xoops.org/modules/xdocman/
http://docs.xoops.org/modules/wfdownloads/viewcat.php?cid=1
https://xoops.org/modules/repository/singlefile.php?cid=43&lid=1223

the Dev Wiki might also have some pointers, you could use.