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 Autism and related subjects shelfmarks:
D 371.9046 Inclusive education
D 371.94 Children with autism - education
D 616.85882 Autism
D 616.858832 Asperger Syndrome