"When do you read the book?"
Translation:Khi nào bạn đọc quyển sách?
First, các bạn is plural, not singular. Đọc sách just mean to read books in general. If you add the classifier quyển, you mean a particular book: the book. And you can choose if you prefer khi nào at the beginning or at the end of the sentence, as long as you don't want to give a past tense touch, in which case you better put it a the end.
khi = when as in "at that time."
khi nào = when as in "at which time or at whichever time." So it is used to ask questions or in place of "whenever." Nào is sort of the "ever" of "khi" (when).
Note that we can say "whenever you're ready, come to see me" or "when you're ready,.." so both khi and khi nào can be used here as well.