John saw not a book, actually, but a scroll, as the description that follows shows. The Father holds in his right hand a scroll which is rolled up and sealed so that it cannot be opened.
The symbolism is similar to that of Ezekiel 2:9. There too the Lord appears with a scroll in his hand, written on the inside and the outside. This scroll had written on it lamentations and warnings and woe. The opening of the scroll that John sees also brings lamentations, warnings and woes as chapter 6 describes.