It turns out that Philip Pullman has a whole other series to his name aside from the deservedly-hyped His Dark Materials trilogy. This is the first title in that series, and it's billed as "A gripping mystery from the depths of Victorian London".
For the most part, it lives up to this description. The book has a decidedly Victorian feel to it - a sort of semi-Gothic creepiness, sinister thieves, young street urchins. Sally Lockhart, the heroine, is a smart and savvy young girl who, having just lost her father, is determined to sort out the mystery surrounding his death and find her own way in the world (despite the machinations of the grouchy old maid of an aunt whom Sally finds herself living with).
While enjoyable, this book didn't quite have the unputdownability I was hoping for, based on other Pullman novels I've read. However, I am still looking forward to reading the other three books in this series, so watch this space to see if the stories get any more absorbing after this.