I've read The Big Sleep before and the thing I didn't pick up on last time is that Philip Marlowe is a guy burdened by his sense of morality and ethics. Everything would be so much easier if he didn't care about what was right, but he does.
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.