The book I want is not on the shelf. What do I do?
The exact procedure varies from library to library, but generally speaking, you can start by reporting missing books in-person at the appropriate Circulation Desk. Staff there can check to see if the item has been transferred to the Harvard Depository, is being repaired, etc. If they can't locate the book, they will direct you to the next step.
For Widener, please let the staff member at the entrance to the stacks know about the missing book. He/she will send somebody into the stacks to search for the book, and if that person is unsuccessful, a tracing procedure will be initiated.