You're right to say that it isn't assumed that we're reading books. In English we don't do that. In Vietnamese, it is just an expression. "Tôi thích đọc sách" doesn't only mean "I like to read books". It can mean that you like reading in general, "I like to read". It's similar to "bạn có ăn cơm chưa?". Literally, we know it means "did you eat rice yet?". However, in Vietnamese speech, it can also mean "did you eat yet?" in general. While Dan and I agree that "read books" makes sense and is a correct answer, we are also saying that just "read" should be an accepted answer as well. This is something DL can accommodate and nuances like these teach us to become even better in Vietnamese. I hope that helps =)
um I tend to disagree. "tôi thích đọc sách" cannot mean to like reading just anything, otherwise a plain "tôi thích đọc" would suffice, although it sounds a little off. I would probably be more specific: "đọc chuyện", "đọc báo", "đọc tạp chí", etc.
as for "ăn cơm", I agree it doesn't usually mean the literal eat rice, but a whole meal. but again, if you mean "have you eaten?", you can simply say "bạn ăn chưa?"
It may be true that to capture the English definite article in Vietnamese you have to use a classifier (quyển sách), but, after doing two third of this course, I have more than passing doubt that the reverse is true. The definite article is not a classifier and does not function like one. The classifiers are not definite articles and do not function as such. There is some overlap but no equivalence between English articles and Vietnamese classifiers.
The plurality is implied by context. The information actually expressed is simply "I read book from morning to afternoon." Unless some specific book has been established by context, the assumption would be that the speaker doesn't read the same book over and over. What can be understood is not overtly expressed. It is the English, not the Vietnamese, that demands overt expression of number as a grammatical category.