The following version of the book was used to create this study guide: Harkness, Deborah. The Book of Life (Book Three of the All Souls Trilogy) Penguin Group, New York, New York, 2014. Kindle AZW file.

Diana Bishop is a witch who has recently discovered she is a weaver, a witch with the power to create spells rather than just following the spells handed down through families and covens. Matthew Clairmont is a vampire who suffers from blood rage, a condition that makes it difficult to remain in control during emotionally-charged situations. The two fell in love, married, and mated. Diana is now carrying Matthew's twins, though most believe it is impossible for a vampire to father children. Despite their love for each other and their children, they are facing danger, partly because the Congregation (the body that governs all creatures) strictly enforces the covenant. The covenant is a rule forbidding creatures from mingling with those outside their group. That means that a witch and vampire are not allowed to marry, let alone have children. Diana and Matthew know there are other problems facing creatures because powers have become diminished over the years and creature populations are declining. They believe the answer lies in a mysterious book sometimes called the Book of Life and held in a library under the title Ashmole 782.

Diana gives birth to twins, a boy and a girl. The boy soon shows signs of being a witch and the girl has vampire traits. All Diana wants to do is settle into life with Matthew, the twins, and her extended family and friends. The problem is that Matthew's vampire brother Baldwin, official head of the family, wants to kill Matthew's great-grandson who has blood rage. Diana and Matthew agree their own children may not be safe from Baldwin in the future. They set out to establish a scion, meaning they would still be under Baldwin's house but with more power to make their own choices. They battle through those steps only to find that another of Matthew's son's, Benjamin, is out of control and has been killing witches to find one that would carry his child. The situation comes to a head when Benjamin tortures Matthew until Diana and others arrive. Diana kills Benjamin and Matthew survives.

Diana finally manages to get the Book of Life and restore its missing pages, making it legible for the first time in centuries. The book has an enormous amount of information but it allows Diana only one brief look before the pages become blank. Diana is surprised that all that information has moved inside her so that answers to all their questions appear in words on her skin and in her eyes. With that knowledge, she can provide an important clue to Chris and Miriam, a human and a vampire who have been researching creature DNA, specifically in the hope of curing blood rage. What they discover is that creatures are not very different after all, and that both Matthew and Diana have daemon blood, meaning there was a daemon among their ancestry. Armed with this information, Diana forces the Congregation to realize their efforts to keep witches, vampires, and daemons separate from each other and humans have had a negative effect on creatures. As the novel closes, Diana and Matthew are settling into their lives, happy to be together and ready to face their future.