If the you have checked the UD Library catalog for a book and don't find it in our collection, your next stop should be the Worldcat catalog.
Search holdings at thousands of libraries, and request a book or chapter to be sent to you via Tipasa/Interlibrary Loan. Arrival times for your item will vary based on its scarcity and proximity to UD, but in general you can expect to wait at least a week for a book to be mailed to you, whereas a scanned chapter or electronic article may arrive via email in as little as a day.