Main Library Circulation staff give directions for where to find books within the building and can assist you in requesting them from other locations. The stacks guide to locations of books in Main and Deering , and a map of the Lower Level Section  can help patrons find books within the building by location and call number. Stacks guides posted are posted near the elevators and there are maps at the entrances to the Main towers. More information can also be found at "How do I find....?"  .
All books in the Northwestern University Library system are listed in NUcat. While books listed with the location of Main collection are housed in the towers by call number, there are a number of separate locations for specialty collections within the Main and Deering buildings. Always note the full location of a call number; this will help you (and us!) locate the book you need.
In the towers, the call number ranges for regular sized books start immediately to the left of the entrance and continue clockwise around the room. Large-sized books in the same range start at the end of the regular-sized ranges, and as a result of the spoke-like layout, are immediately to the right when you enter the tower. Large-sized books are identified by a letter "L" at the start of the call number. There is a color-coded map of at each tower entrance.
How long it takes for a particular item to be reshelved after it is returned varies. If you can't find a book that NUcat lists as "Not checked out" or "Returned on [date] [time]", check the entrance of the tower for book carts that may be waiting to be reshelved. If the book is not there, check with Circulation to see if it is on a cart that hasn't yet made it upstairs.