WorldCat is a world-wide catalog of books. It's possible to request books from other libraries (beyond Mobius libraries) through WorldCat. WorldCat functions in the same way as the WU catalog; you may search by keyword or subject.
Just as with the WU catalog, the subject headings are listed as links at the bottom of the record. I highly recommend exploring WorldCat to see what other sources are out there.
We have access to two WorldCat interfaces. One is free, one is subscription. I prefer the subscription version. Links are below:
If you rely only on keyword searching, you may miss some excellent, relevant sources since keywords, like the tags applied to photographs, are not standardized.
Searching by subject, on the other hand, allows you to tap into a system of standardized, controlled, and organized vocabulary and thus more precise and comprehensive searching.
An example: If you were researching the portrayal of monsters in literature, a keyword search of "literary monsters" would yield many relevant results and some irrelevant ones too. After checking a few relevant-looking titles, you might find the subject Monsters in Literature, as well as related sub-headings.