Why are 'e-books' not a category when doing a catalog search?
The downloadable items are slowly, but surely being integrated into the main catalog. You can search for "EBOOKS" and "EAUDIO" by doing an advanced search.
In the dropdown menu for material type you are able to search by EBOOKS, EAUDIO, and EVIDEO. It's not perfect at this time, but one way you can find those items on the general catalog.