The Copy/Holding information on the catalogue tells you if the item you want is in either of the separate collections - the Reading Room Collection (on level 1) or the Reference Collection (in front of the help desk). All other items are found on levels 1-4.

Within each collection and in the main book sequence, the books are arranged by shelfmark. 

Why do some books have number shelfmarks and others have alphabetical ones?

We have two subject sequences. The majority of the stock is arranged in number order

e.g. 320 = politics
320.941 = British politics
320.973 = American politics

Some books have been classified by a scheme that uses letters for the main subject areas

e.g. YB English Literature
YD English Literature in the 15th & 16th century
YF Shakespeare

To find out which shelfmarks are located on which floors, use the Finding your way around part of this module.