asterisk users - Mar 2006 - Information to program a new driver for Asterisk

I'm interested in developing a new channel driver for a thrid party 
telephony card for Asterisk. Is there any "official" document that 
explains how to do this? We've been looking the doc/channel.txt and 
doc/modules.txt in the source, but that's not a very complete source of 
info :)

Thanks a lot for your attention.

-- 
Atly.
Alvaro Palma