My take on the "book" that was sealed with seven seals is that it is the Lamb's book of life (or life and immortality). You'll notice in verse 6 that the book was unsealed by Jesus after He was slain. Now compare this with Rev. 13:8.

Rev. 13:8 And all that dwell upon the earth shall worship him, whose names are not written in the book of life of the Lamb slain from the foundation of the world.

Peace