There are a number of good docs available on the web but that are too large to include in the base tarball. A brief list:

The HOWTO: the basic intro to legOS.
http://legOS.sourceforge.net/HOWTO/

API documentation:
http://legOS.sourceforge.net/docs/CAPI/

C++ API documentation:
http://legOS.sourceforge.net/docs/CPPAPI/

kernel docs: intro to the kernel, covering how it works.
http://legOS.sourceforge.net/docs/kerneldoc/


