Already know what book you want? The MU Libraries Book Finder will automatically search first for available copies of it on campus, then within UM libraries, then within the state of Missouri.
Use the MERLIN catalog to find MU Libraries-owned books by author, title, subject, etc.
Use the MOBIUS catalog to find books in other Missouri libraries.
Use Worldcat to find books beyond what's available in Missouri.
Use Google's Advanced Book Search to search within a huge corpus of books. When you find books, look for the Find In a Library link to locate a copy.
Use HathiTrust to do similar searching to Google's Advanced Book Search, but with better connections to library holdings plus the ability to build personal "collections" of books that you can cross-search.