。
Do you agree with what he says in the book?
Literal
Book [inside-の中で] he [subject-が] saying [thing-こと] [to-に] you [topic-は] agree [is-です]-[question-か]?
の中で ('in, inside, within,' literally 'in the middle of') is the standard way to express 'in [a book/story/speech/etc.].' The embedded clause 彼が言っていること ('the thing he says') is nominalized with こと and made the target (に) of 賛成する ('to agree with'). A textbook-style but natural grammar question.