The second adventure of Rhiouw and the feline wizard getekeepers who first appeared in THE BOOK OF NIGHT WITH MOON (although Rhiouw's predecessor appeared in SO YOU WANT TO BE A WIZARD).
If you loved TBONWM and Duane's Nita and Kit books, you'll probably at least like this one. If I'm less than totally wowed it's partly because the incidental characters are rather boring. The confrontation that sets up the penultimate "surprise-ending" chapter comes out of left field and makes the climax less satisfying than the previous sequences of Victorian London and Arhu's reconciliation with his past.
All that being said, it's a good read especially if you're already a Duane fan (as I've been for years). NB: the book appeared in the UK as ON HER MAJESTY'S WIZARDLY SERVICE (for a change, the US title is better, more appropriate).