Tafsir of Al-Qalam 68:37

Or do you have a book descended from the heaven?

In it—meaning in the book; the prepositional phrase is linked to His saying, the Exalted: you study, meaning: you read therein. The sentence is an attribute of "a book." It is also permitted that "in it" relates to a predicate-related element, or it is the attribute, and the pronoun refers to the judgment or the command, while "you study" is an initiating clause or a circumstantial qualifier for the pronoun of address.

And His saying, the Exalted: