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 333.7 Renewable energy
D 532 Fluid mechanics
D 621 Mechanical engineering (general)
D 621.8 Machine engineering
D 629.1 Aerospace engineering
D 629.4 Astronautics
D 629.892 Robotics