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.
Here are some useful Counselling shelfmarks:
D 155.937 Bereavement counselling
D 158.3 Counselling psychology
D 361.06 Counselling
D 371.4 School counselling
D 616.8914 Person-centred counselling
D 616.89143 Gestalt psychotherapy