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 Safety and related subjects shelfmarks:
D 158.7 Occupational psychology
D 333.7 Environmental policy
D 347.49 Health & safety - Law & legislation
D 613.62 Occupational health
D 614.852 Health & safety - Management
D 620.0045 Risk assessment
D 658.382 Workplace health & safety