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 Journalism and related subjects shelfmarks:
D 070.19 Television news
D 070.4 News journalism
D 072 The press
D 079 Newspapers - history
D 302.23 Mass media
D 302.231 Social media
D 820-4(09) Writing