Adam Clarke on Revelation 5:1

A book written within and on the back side - That is, the book was full of solemn contents within, but it was sealed; and on the back side was a superscription indicating its contents. It was a labelled book, or one written on each side of the skin, which was not usual.

Sealed with seven seals - As seven is a number of perfection, it may mean that the book was so sealed that the seals could neither be counterfeited nor broken; i.e., the matter of the book was so obscure and enigmatical and the work it enjoined and the facts it predicted so difficult and stupendous, that they could neither be known nor performed by human wisdom or power.