The IOP Architecture has never been documented but it has been mentioned that the IOP Manager and implied IOP Architecture was never documented because it was thought that this was a propriertary advantage. The IOPs are mentioned breifly in various tech notes and the Guide to Macintosh Family.
The best documentation is presented in a description from Apple's Technical Support reformatted locally:
Apple Tech Support Description
I have been also playing with MacsBug trying to reverse engineer parts of the IOP communication: