Tales From the Black Chamber — Bill Walsh

When one of her clients turns up dead after buying an apparently unremarkable 16th century breviary, antiquarian book dealer Anne Wilkinson is suddenly swept into a world that she had previously considered imaginary. An esoteric world of occult spells and invocations, of cryptic texts and secret doctrines, a world where necromancers spy through mirrors and armed assassins blow up her office and try to gun her down.
Before she knows it, Anne is recruited by representatives of a secretive government agency established by Calvin Coolidge to fight demons, vampires, werewolves, and other supernatural beings -- and finds herself caught up in the pursuit of a renegade priest who seeks the power to unleash one of the darkest forces of the past into an unsuspecting present. When the unthinkable occurs, only Anne and a handful of books stand between humanity and the end of the world. Can her intrepid band of scientist-librarians steeped in occult lore and schooled in NSA-level techniques of surveillance, explosives and heavy weaponry prevent the demon known as Abbadon from immanentizing the eschaton? You'll have to read to the end of this page-turning thriller to find out!