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.
Useful Psychology and related subjects shelfmarks:
D 150 General psychology
D 151 Intelligence
D 152 Sensory perception
D 153 Cognitive psychology
D 154 Learning and behaviour
D 155 Developmental psychology
D 156 Comparative psychology
D 158 Applied psychology
D 302 Social psychology
D 616.89 Clinical psychology