A slang term for a homosexual man's anus, which has been stretched so much that it no longer can close. The world's most famous "book of life" is that of goatse.
Damn, my ass is a book of life tonight, jeez.
God's book in which the names of everyone who has allowed Jesus into their hearts during their brief stay on earth are written for all eterinty.
You can go to heaven provided only that your name is written in this book. To get these two (three, four, five, sixteen, however long your name is) words of text permanently written into this obviously huge book, you must accept Jesus into your mind heart, and soul.
The book of life has the billions of names of the people who accepted Jesus.