Why must be books (plural)? Can't it be book?
If it's singular, then it must be used with a classifier (quyển/cuốn). Nevertheless, saying "tôi mua sách" is quite vague. Will expand on this later.
they do not said that is : Tôi mua những cuốn sách
1) to buy
From Wiktionary: https://en.wiktionary.org/wiki/mua
Read this post about classifiers: https://www.duolingo.com/comment/17166436
If there are no classifiers, it refers to a general statement. In this case, books in general rather than a specific book.