The sealed book in Revelation 5 is the Lamb’s Book of Life.
The sealed book of Revelation 5 symbolizes things that are not understood. Several parallels identify it as the book of life. This is supported by Revelation 12. Revelation 5 and 12 describe the same crisis and the same events. What Revelation 12 adds is that the crisis is about God’s judgments, which are contained in the book of life.