First confirm the item's status in the catalog is AVAILABLE and note what LOCATION is indicated.  Other than MAIN, books may be on Main Reserve or Art Reserve, in Periodical Room, or in the Art or Music Library  among other library locations.

This guide to locations explains what book call numbers are located in what rooms.  This map of the library includes the room numbers. If you are having trouble you can go to the circulation desk and the Library or Student Staff will assist you in locating your items. 

If the book is indeed missing, submit this  not on shelf form. Library staff with search and respond, usually within 1 weekday.  When the LOCATION of the item says MISSING in the catalog, you can use of the interlibrary loan options to request another copy.