Library books are arranged by the Dewey Decimal classification scheme whereby each subject is represented by a number. You will find books on the same subject will have the same number. The shelfmark is preceded by the letter D and will be followed by 3 letters, which are usually the first 3 letters of the author's name.
D 327 International Relations and Diplomacy
D 940 European history
D 956 Middle Eastern history
D 973 American history