Library books are arranged by the Dewey Decimal classification scheme whereby each subject is represented by a number. You will 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.
Useful Translation and related subjects shelfmarks:
D 418.02 Translating and interpreting
D 420 English language
D 425 English grammar
D 440 French language
D 450 Italian language
D 460 Spanish language