FWIW, my opinion of the complete maya programming books is extremely low. If I was being charitable, I'd say the book is better than nothing, but if I was being honest I'd say it's an awful book that's filled with mistakes from cover to cover. Don't bother buying both volumes. The second volume is effectively the first volume, but with a slightly more useful layout (although most of it is still utterly wrong, but hey...)
Mel isn't really for plug-in development as such. It's got a kind of dual role in Maya.
1. To facilitate Maya's undo mechanism.
2. To provide a set of methods to define the GUI of Maya (which can be ripped back to the bare minimum if needed)
For motion capture, depending on what your aims are, mel is potentially the worst possible tool available! The one thing mel is very bad at, is processing tonnes and tonnes of motion data (make sure you disable undo if you are attempting this!). If you are simply wanting to write some custom tools that simply call some export/import plug-in, then it's going to be very useful. If you find yourself doing lots of computation, think again ;)
If you do need to process a tonne of data, you'd be better off utilising the C++ plugin API (see here for further info:
http://nccastaff.bournemouth.ac.uk/jmacey/RobTheBloke/www/ ). Typically you'll also need a little bit of MEL to expose some aspects of your C++ code (examples on that site too).
You can also use python, which is *a bit like* a half way house between MEL and the C++ API (It can do everything MEL can do, and exposes most of the C++ API too). I'd probably suggest this route to be honest, for a number of reasons:
1. Tutorials on the python language are 10 a penny, unlike MEL.
2. Motionbuilder (which as you will know, is a much better app for processing mocap data than maya's limited functionality) has a python API too....
3. Since you have access to the C++ API, you can bypass MEL's undo layer, which will give a significant performance boost (although make sure you adhere to the best practices for the C++ API. The API is not as obvious as it first seems. Beware of the gotchas!
http://www.creativecrash.com/forums/api/topics/new-to-the-api-then-read-this-33 ).
I'd also point you in the direction of cgtalk.com. The forums there are usually the best place to ask MEL/API questions.....
