An entry from Open Access News discusses free online books available from the U of California Press and National Academy Press. I knew about the latter, but not the former. Didn't see much to recommend to my particular group of distance students, but you might find something relevant for yours. Apparently UC Press offers MARC records and PURLs as well. Good to know.
Here's a list of all the available U of C Press books by subject: http://texts.cdlib.org/ucpress/subjects_public.html