My 10yr old...erm, well he was 8 when he started reading them, is a big fan of the Barnaby Grimes series
Barnaby Grimes: Curse of the Night Wolf
Barnaby Grimes: Return of the Emerald Skull
Barnaby Grimes: Legion of the Dead
Barnaby Grimes: Phantom Of Blood Alley
by Paul Stewart and Chris Riddell all set in a psuedoVictorian London and since has gone on to the Edge Chronicles by the same chaps.
I've also read them, purely to see if they are suitable y'unnerstand, and would happily recommend them.