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Shadow of Night (All Souls Trilogy #2) by Deborah Harkness

Shortly after Diana Bishop and Matthew Clairmont timewalk to London, 1590, they discover that the past may not provide a safe haven after all. Reclaiming his former identity as poet and spy Matthew Roydon, the vampire falls back in step with a group of radicals known as the School of Night who share dangerous ideas about God, science, and man. Many of his friends are unruly daemonsn - the creative minds of the age who walk the fine line between genius and madness - including playwright Christopher Marlowe and mathematician Thomas Harriot. Matthew, himself, is expected to continue to spy for Queen Elizabeth, which puts him in close contact with London's cutthroat underworld. 
Together, Matthew and Diana scour the bookstalls and alchemical laboratories of London where they follow the elusive trail of Ashmole 782 - and search for the witch who will teach Diana to control her powers