Shadow of Night by Deborah Harkness

Shadow of Night : Paperback : Penguin Putnam Inc : 9780143123620 : 0143123629 : 28 May 2013 : The #1 "New York Times"-bestselling sequel to "A Discovery of Witches" picks up from the first novel's cliffhanger ending. Diana and Matthew time-travel to Elizabethan London and are plunged into a world of spies, magic, and a coterie of Matthew's old friends, the School of Night.