Update: Robert Bateman wrote a similar document [10] like this one. Have a look at it.

Unfortunately there isn't much documentation out there. Maya's help is more or less equivalent to Java-doc (so only helpful, when one needs information about a specific function or parameter), and the existing books are mostly about using Maya itself, or writing MEL-scripts. The only book about Maya's API I found is "Complete Maya Programming" [7], which promised to be helpful. I was quite disappointed though. The book might be nice for writing other kinds of plugins, but did definitely not help in writing the exporter.

Still: there are helpful resources. GameDev.net [9] provides an example of a complete stand-alone exporter, including animations. It hides some complications one usually encounters, but it's still a great site. Other sites [6], [8] contain FAQs for Maya-plugins (very helpful).

Finally there are some open-source exporters (or close too;). MS's exporter found in the DirectX SDK [3] is quite complete, but mostly unreadable. ATI's normalmapper [1] contains an exporter for static meshes (so not very exhaustive). Quake-Rally's exporter [5] has been recently updated, and I've to admit: I don't know how helpful it is now. And finally neo-engine's exporter [4] and Ogre3D's exporter [2] are both very helpful (and in some parts similar).