"Do they sell books?"
Translation:Vendent-elles des livres ?
Les and des always give me trouble as it's not always as simple as 'des' meaning 'some'. I generally think of it as 'des' = some of a group and 'les' = all of a group but I think that's too vague to use in this example.
The best way I can describe it is that 'des' can mean 'any' as well as 'some'. So this sentence is essentially asking if a shop sells 'any' books and not a general assessment of books. So to put books in the definite article would be something like 'Les livres sont bons'
Perhaps this may help explain it a little better: "The plural indefinite article, des, is used when referring to more than a single entity. It is not used, however, when general statements are made about a group - statements that are meant to refer to all the entities that make up that group. This is the role of the definite article as we shall soon see." http://www.languageguide.org/french/grammar/articles/
Being a native English speaker the difference between the use of articles in French and English confuses the hell out of me.
I hope my examples/explanations aren't wrong but at least the link I provided may help.