This is a supplemental book that comes right before the last one in the Blue Bloods series. It isn't essential to read, but it definitely helped me with some background I needed before everything in the finale went down. Overall, I really liked it, and I definitely appreciated how it tied two parts of the world together before the last book. I also think that it's interesting how de la Cruz tied The Witches of East End story line into this one a little. I would definitely recommend reading it before finishing the series!