Laws of Witchcraft by C.J. Archer

When Gavin travels to Edinburgh with his good friend Oscar Barratt, he’s eager to acquire a centuries-old book on witchcraft for the soon-to-open Glass Library. Even with ruthless railroad magnate John J. Defoe in the bidding, the victory comes easily. Perhaps too easily.
Hours later, the book is stolen.
A single clue links the theft to the recent abduction of two women, both magicians. Desperate to recover the book, Gavin and Oscar find an unexpected ally in Defoe’s enigmatic assistant, Miss Wheeler. But as they delve into the mystery, they uncover far more than they bargained for: a secretive sect, a descendant of the Witchfinder General, and forbidden love letters.

With time running out for the kidnapped women, can the unlikely trio untangle the lies from the truth before the magicians—and the book—are lost forever?