I've been reading Hammett in tandem with Chandler, in pairs, and I find they go well together. The
The big difference, of course, is that Nick has Nora. He's not alone. On a pure entertainment level this allows for the witty repartee that has made this book such a favorite of so many for so long; I can see why there were so many attempts to film sequels - you just want to stay in Nick and Nora's company as long as you can. The solitary Spade or Op (or Marlowe) banters, but usually in a semi-confrontational way with at least semi-antagonistic interlocutors. Nick can joke with Nora in perfect ease.
On a deeper level this is an incredibly significant change. The hardboiled private eye is defined by his solitude. This is true of Spade, the Op, Marlowe, Easy, O, and everybody else I can think of. The genre is all but synonymous with a moral solitude - me against the world, me the last oasis of sanity in a desert of evil and absurdity, me. The private eye may have allies, but only of convenience - cops are usually looking for an excuse to bust his balls, clients are usually lying to him, friends are either powerless or untrustworthy. The noir thematizes solitude.
But Nick has Nora. Now, a trusty sidekick and confidante is not necessarily a rarity in detective fiction more broadly: indeed, it's an archetype. Holmes has Watson, and that pattern has held true in about any number of recent mystery series on American TV alone. But that world is a brighter one than the noir world we're talking about here. The typical noir admits of no such companionship.
What The Thin Man does so successfully, then, is to bring in this rewarding, heartening sense of companionship while maintaining the hardboiled nature of the story and the world it's set in. We don't doubt that the streets in this book are as mean as those in Red Harvest or The Maltese Falcon. We don't doubt that corruption and betrayal are just as rife, just as likely, as in those books. But we - inhabiting Nick - aren't as touched by them, because of the vibrant, capable, knowing, and redeeming presence of Nora.
Not that she's an angel - that would destroy the mood. Rather, she's us, the readers: we're inhabiting Nick's perspective, but our position is actually Nora's. Left out of Nick's calculations, discovering his deductions when he chooses to speak them, and most of all learning about his world along with Nora - with enthusiasm and salacious glee. She's less a sidekick then a sponsor and fan. Hammett allows us into the book that way, but also allows Nick escape. It's still properly his world, this morass of low motives, but he doesn't have to live in it anymore. Nora has delivered him.