Circulation staff on the first floor of the Main Library are able to double-check call numbers and give directions for where to find books within the building, or to request them from off-site locations. Handouts are available at the Circulation desk to help patrons find books within the building by location and call number, including a stacks guide to locations of books in Main and Deering [1], and a map of the Lower Level Section [2]. There are stacks guides posted in and around the elevators, and maps at the entrances to the Main towers. More information can also be found at "How do I find....?" [3] in our Guides and Tutorials.
NUcat holds the records for all books in the Northwestern University Library system, including several specialty libraries on both the Evanston and Chicago campuses. 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 when you are writing down a call number; this will help you (and us!) locate the book you need much faster.
Main Library books in the same call number range are shelved separately according to size. 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 capital letter "L" at the beginning of the call number. There is a large color-coded map of each tower room posted near its entrance.
The length of time it takes for a particular item to be reshelved after it has been returned varies depending on a number of factors. If you can't find a book that NUcat lists as "Not checked out" or "Returned on [date] [time]", check the entrance of the room for any book carts that may be waiting to be reshelved. If this doesn't work, check at the Circulation desk to see if it is on a cart that hasn't made it upstairs yet.
Links:
[1] http://www.library.northwestern.edu/find-materials/specific-materials/book-location-guide
[2] http://www.library.northwestern.edu/sites/www.library.northwestern.edu/files/pdfs/lower_level.pdf
[3] https://www.library.northwestern.edu/node/1343