Elemental Detectives by Patrice Lawrence

Marisee and Robert are the Elemental Detectives chasing the clues to avoid catastrophe: they must face the ghosts of Hyde Park, the monstrous coiled snake of the Serpentine, and a whole host of other fantastical creatures in their mission to stop the Shepherdess and prevent London slumbering for all eternity...

I loved this one - it's action packed, pacey and with a setting in the underground rivers of London it's refreshingly different. I will read anythig Patrice has written and this one didn't fail me!