EMAPI will go to Exchange, as will ODI. These are not trivial protocols.
One reason to go through Outlook is that you avoid all the log-in issues (assuming you turn off the Security dialogs). You can also create a "user" (we named ours Maxwell Scheduler) that is a delegate for real users' calendars so it could act as a PA.
I found a company (in Romania?) that had a lot of information and a toolkit for Exchange work.
slipstick.com is a good clearing-house for Outlook and Exchange information.
Developing Applications using Outlook 2000, CDO, Exchange, and Visual Basic by Raffaele Piemonte, Scott Jamison has useful information. I just got Rizzo's book, so I don't know how it is.
Cheers