Shadow of Night by Deborah Harkness

From Library Journal: "In the wake of Harkness's best-selling debut, A Discovery of Witches, fans have been im-patiently waiting to discover where Matthew, the vampire geneticist, and Diana, the witch historian, landed when they "stepped into the unknown" at that novel's cliff-hanging close. Now in Elizabethan England, they are hunting an enchanted alchemical manuscript. At the same time, Diana must continue her magic education and Matthew reunites with friends, including Christopher Marlowe and Sir Walter Raleigh. New readers should start with the first book, as the sequel seamlessly continues the adventures of the scholarly pair and because of the author's detailed world building and complex story line. The characterizations of 16th-century Oxfordshire with a paranormal twist (Marlowe is a gay daemon, for example) add a historical richness to the epic, while the plot sets readers up for the culminating struggle anticipated in the trilogy's final book."