Why do you translate "sach" suddenly as books. I know that in real Vietnamese it can have the meaning, but Duolingo is quite inconsistent in its own use of "mot quyen sach" and "sach" and "nhung quyen sach".
Each phrasing of words yields different contexts. The phrase "tôi đọc sách" = "I read books", and it implies that you're reading books in general. "Tôi đọc một quyển sách" = "I read a book" and I'm sure we all understand its context. "Tôi đọc những quyển sách" = "I read the books" which means that you're reading a specific set of books. I hope that helps? I get that DL is imperfect and it can be frustrating to learn from sometimes. I think they're just trying to hammer phrasings and repetition into our heads. However, it is just one of the many ways we could learn languages.
I'm sorry but "tôi đọc sách" does not necessarily the general and plural books. it could perfectly be translated as just a book, depending on the context. just saying.
If someone asks about my daily routine and it is my practice to read at five thirty, I can answer, in perfectly understandable English, "I read a book at five thirty. " No one would take me to mean that I read the same book every day. I could answer in the plural and be understood in the same sense because no one would assume that I read books literally simultaneously, or I could simply say, "I read at five thirty" and let my listener assume I read books. Alas, DL seems to think that Vietnamese idiom somehow governs English idiom. This leads to normal English being called incorrect by DL. It is frustrating to have my native English corrected when I understand the question and answer accordingly.
Vietnamese nouns don't seem to have number by themselves. It seems that if there is no plural indicator, like các or những, number is understood from context. But, since I'm not a native speaker, I don't speak with any authority here.
Dan is correct saying it depends on the context. if I say "tôi đọc sách", you can translate either into the singular "I read a book" or the plural "I read books". if I say "tôi thích đọc sách", it might be better to use the plural "I like reading books" rather than "I like reading a book". on the other hand, if I say "tôi đang đọc sách", it is safe and more probable to presume that "I am reading (just) a book" and not "I am reading (many) books".