I finally found a way to really get rid of the very large text books I wanted to delete. Simply deleting them from ibooks did not work. The usage still showed that these "deleted" text books were taking up 1.9 GB of space in ibooks. Reseting the cache in itunes (preferences, advanced) didn't work either.
I simply downloaded a much smaller book and synced the ipad with itunes. During the sync process, the new data finally replaced the old data (including the huge text books that were sitting in some cache). Poof! They are finally gone and the usage now shows 9.4 MB instead of 1.9 GB. Yaay!
Try it .. it may work for you too.