I've now read all of Hammett's novels. It isn't a huge feat, there are only five. Still, it was fun.
I don't think this one gets enough recognition. The structure is weird. Probably because it was serialized before being printed as a whole novel. But it kind of has it all; ghosts, religious cults, bumbling police, mad scientists, diamond thieves, a girl with elf ears. Everything. I liked it.