"Ich habe wenige Bücher."
Translation:I have few books.
The sentence means 'few books', not 'a few books', this must be a mistake in the dictionary of duolingo. The meaning of "I have a few books" is not the same as "I have few books". The first sentence just says that you have a small number of books, whereas the second emphasises that you don't have many books, it suggests an expectation that you would have more books. In german you would say something like 'Ich habe einige Buecher' for "I have a few books".