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 chemistry shelfmarks.
D 543 Analytical Chemistry
D 546 Inorganic Chemistry
D 547 Organic Chemistry
D 574.19 Biochemistry
D 615 Pharmacology and therapeutics
D 660.2 Chemical engineering