Shadow of Night by Deborah Harkness
Book Two of the All Souls trilogy plunges Diana and Matthew into Elizabethan London, a world of spies and subterfuge, and a coterie of Matthew's old friends, the mysterious School of Night. The mission is to locate a witch to tutor Diana and to find traces of Ashmole 782, but as the net of Matthew's past tightens around them they embark on a very different journey, one that takes them into the heart of the fifteen-hundred-year-old vampire's shadowed history and secrets. For Matthew Clairmont, time travel is no simple matter; nor is Diana's search for the key to understanding her legacy.
Read the first in the series, if you haven't already: A Discovery of Witches by Deborah Harkness