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.
e.g. D 361.32 KOP
Here are some useful Social Work and Social Policy shelfmarks:
D 300 Social sciences (General)
D 361.3 Social work
D 361.61 Social policy
D 362 Social welfare problems & services
D 364 Criminology
D 909 Human geography