No unclean person, anyone who commits abominations or lies can enter. Only those whose names are written in the lamb's life book can enter. (Revelation 2 1:27 and KJV)
The book of life and the book of lamb mentioned in these two verses are actually a book of life. Because the judgment before the White Throne was presided over by Jesus (the Lamb), the book of life is also called the book of life of the Lamb.