Fourth in a series (The Merchant of Death, 1995, etc.) by Grace (a.k.a. P.C. Doherty) built around Kathryn Swinbrooke, physician and herbalist in 15th-century Canterbury. Edward of York and his beautiful Queen Elizabeth Woodville rule in the aftermath of civil war, but Elizabeth feels threatened by the spells and secrets held by her necromancer (and blackmailer) Tenebrae in his journal--The Book of Shadows. When Tenebrae is found murdered in his black mansion in Canterbury, Kathryn is called to the scene. She finds a locked-room scenario in which those present at the time of the magus's death, composed of a group from a London goldsmith's guild whom Tenebrae saw one by one, appear innocent, but the infamous Book of Shadows has vanished. Kathryn's Irish love, Colum Murtagh, the King's Commissioner in Canterbury, is under pressure from Queen Elizabeth to recover the book and identify the murderer. Together, they talk to everyone involved--from Tenebrae's servants Morel and Bogbean to Sir Raymond Hetherington, physician Brissot, rich widow Dauncey, clerk Fronsac, and others of the London party. When members of that group begin to turn up dead, Kathryn returns to Tenebrae's house, risking her life but eventually coming up with an inspired solution. The author's medieval mix as before--repetitious catalogues of the city's sights, sounds and smells; much good-natured chatter with Kathryn's lusty cook Thomasins; a surfeit of local ``characters'' and ailing patients, and an unconvincing killer. Of possible interest to devotees of the period--for others, mostly tedium.