When looking for books, you'll want to search in the Library Catalog or Scout. When looking for a specific book, the catalog works a bit better. When looking broadly for books on a topic, you'll want to search in Scout, which includes both ebook and print results.
If you need a book we don't have, there are a few options:
Contains all the records cataloged by OCLC member libraries. Offers millions of bibliographic records. Includes records representing 400 languages.