This was a fabulous, exciting read for 10+. I'd recommend it to fans of Philip Pullman's The Ruby in the Smoke and sequels; like Sally Lockhart, Penny is brave and resourceful, and it has a pleasing classic Victorian adventure-mystery feel. In a Twitter conversation about Sally Lockhart, Matt Finch said that what he liked about the books was that Sally can be a "kick ass" heroine without having a stupid boy as a foil, and I liked that about Penny, as well. Penny doesn't have to disenfranchise Alfie to be a strong agent; it's a book that I would happily recommend to boys as well as girls.
A word about Nosy Crow: I am so impressed by the consistently high quality of their list. I have not yet read one of their list that I didn't enjoy. Wonderful books for all ages.