I read couple of related blogpost's related to MAPI documentation. You can start reading from Sgriffin's detailed information regarding the MAPI documentation. Also i found couple of related blogpost's from mstehle and Angela.
You can end up by reading the article posted by Thom Randolp, which talks about the MAPI documentation. Per him, "... based on some discussions we had a while ago, the folks in Office have been putting a LOT of time and effort into improving the MAPI material, and bringing it up-to-date for Office 2007. Well, they’ve finished their work, and the improvements are substantial! Check out the new content on MSDN Library at: http://msdn.microsoft.com/en-us/library/cc765775.aspx. We plan to have a redirection page under Exchange Server for about six months, to remind you where the stuff went..."