If Bobst or an NYU Consortium Library (New School, Cooper Union, IFA, etc.) does not own a book that you need, you can request it from Interlibrary Loan.
Most mathematics books are in the QA section of libraries organized by the Library of Congress system. To browse books on the shelf you can go to Bobst Library and head to the 9th floor (you can find a map of this floor on the Call Numbers and Floor Maps of Bobst Library page) or go to the Dibner Library.