It's possible that Princeton might not own a specific book or other kind of material or the item is in use or missing. If another library is willing to lend, PUL can obtain a copy to borrow at no cost to you. There are two ways to get books that Princeton doesn't own.
Borrow Direct: This is a consortium of Ivy+ institutions. You can search the site as if it were a catalog and place the request yourself. You'll get an email when the book is delivered to the library. You can check out Borrow Direct items for 16 weeks (no renewals permitted).
Interlibrary loan: This service is broader than the Ivy Plus consortium and allows us to contact libraries worldwide for items. You fill out a form and our staff do their best to locate the item for you. The borrowing period is determined by the lending library.
If you need an article or book chapter in an item PUL doesn't own, try the Article Express service. The turnaround is usually pretty rapid.