Books about the same subject are assigned the same shelfmark number. You'll find the number on the spine of each book. Here is a selection of shelfmarks for Speech and Language Therapy:
Speech and Language Therapy (General) 616.855
Clinical practice 616.85506
Aphasia 616.8552
Dyshagia 616.31
Phonetics 421.5
Language 410
Psycholinguistics 401.9
Ethics 174.296855
