The Metasploit team provides some documentation on how to develop exploits, but other than looking through the sourcecode (which is annoying considering how much time I spend going from this file to that file) is there any other semi-comprehensive reference that would make it much easier to learn about the framework? Such as discussing common functions, but in more detail than the documentation found on Metasploit's website.
They have video series on developing exploits with metasploit here http://www.securitytube.net/groups?operation=viewall&groupId=0
Here are some other links http://www.offensive-security.com/metasploit-unleashed/Useful_API_Calls
Also i recommend looking at exploits similar to what you want to do to see how they work.
The last chapters of the excellent book "Metasploit: The Penetration Tester's Guide" are dedicated to Metasploit development.