Only one? But there's so many good ones! Of course, many are part of a long series of books. How about a nice stand-alone for a change?
Lucy's Blade by John Lambshead
Sir Francis Walsingham, Queen Elizabeth's Secretary of State, was the greatest spymaster the world had ever seen. But when he asked Dr. Dee to summon a demon the result was unexpected, especially for his orphaned niece Lucy. Sir Francis' duty as her guardian was to find Lucy a suitably aristocratic husband, not to let her fight demons and witchcraft for the Queen's Secret Service. But his—and Lucy's—duty to protect Queen and country from enemies both natural and supernatural kept getting in the way. And so did all those demons . . .