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 mathematics and statistics shelfmarks.
D 510 Mathematics
D 512 Algebra
D 513 Arithmetic
D 514 Topology
D 515 Analysis, calculus
D 516 Geometry
D 519 Probabilities and applied mathematics
D 519.5 Statistics