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 620.4162 Ocean engineering
D 621.3121 Renewable energy
D 622.3382 Petrolium engineering
D 623.81 Naval architecture
D 623.82 Shipbuilding