"I need books" means that you need books in general, e.g. "I need books to read while I am on holiday" "I need a book" (no 's', since 'a' is always singular) means that you need one book - e.g. "I need a book to read on the train otherwise the long journey will be very boring" or "I need a book about geometry before I can do my maths homework" "I need the books" means that you need some specific books, e.g. "I need the books on the reading list the professor gave us" (you can also say "I need the book" if you need one specific book)