Shadow of Night By Deborah Harkness

Picking up from A Discovery of Witches’ cliffhanger ending, Shadow of Night takes Diana and Matthew on a trip through time to Elizabethan London, where they are plunged into a world of spies, magic, and a coterie of Matthew’s old friends, the School of Night. As the search for Ashmole 782 deepens and Diana seeks out a witch to tutor her in magic, the net of Matthew’s past tightens around them, and they embark on a very different—and vastly more dangerous—journey.