Library books are arranged by the Dewey Decimal classification scheme whereby each subject is represented by a number.
You wil 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 pharmacy shelfmarks.
D 615 Pharmacology and Therapeutics
D 615.1 Drugs
D 615.4 Practical pharmacy (e.g. dispensing)
D 615.6 Methods of medication
D 615.7 Pharmacodynamics
D 615.9 Toxicology (poisons and poisoning)
D 616 Diseases