Angela just posted an article on extending Outlook on her blog, and an article on picking the right API on the MSDN. Both articles make several mentions of MFCMAPI and are thus worthwhile reads. The MSDN article is an especially good one-stop overview of all the APIs and technologies available in Outlook, with special attention paid to details such as managed vs unmanaged code, bitness, and threading
Note that both articles are written very much from Outlook’s point of view. The presumption is that Outlook is part of your solution. This may not necessarily be the case. If you are considering server side solutions, your top choices will likely be either Exchange Web Services (EWS) or the MAPI download, though there several other Exchange based APIs which you can read about in the Exchange Server 2010 SDK.