In the first book, Diana Bishop, witch, and Matthew Clairmont, vampire, meet and fall in love. It’s forbidden. You get a little taste of why. In the second book, they travel to Elizabethan England to find Diana a teacher, because none of the present day witches know much about her power. While they’re there, Diana becomes pregnant, which should not be able to happen.
So, in the third book, we have to fully explore exactly why the creatures made a Covenant to not interbred, and get that repealed, fully explore what Diana and Matthew’s children will be (it’s twins), delve into creature genealogy, and set up a new family with Matthew as the head, because vampire politics are murky.
Apparently, someone is developing television series based on this series. This book is going to have to play out over a couple seasons. It’s absolutely fascinating, and I really enjoyed it, but I’m torn between respecting that the author actually stuck to the trilogy format (aka not going the Robert Jordan or George RR Martin route of ever expanding series with no foreseeable end), and wishing she’s spaced it out, and we’d gotten more.