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 shelfmarks.
D 551 Geology and hydrology
D 624.1 Structural engineering and underground construction
D 625 Engineering of roads and railroads
D 628 Environmental engineering
D 690 Construction of buildings