"You write books."
Translation:Tu écris des livres.
Ask yourself: can anyone write all books as a category of objects?
"Books in general" is "les livres" in situations where all and any books are considered, as in:
- Books are my best friends = Les livres sont mes meilleurs amis.
Take a closer look at this sentence: In "you write books", "books" is only the plural of "a book". Therefore, the plural of "un livre" is "des livres".