Shane, the protagonist, loses a high paying salesman job to a failing competitor after she makes a deal with Perdition Investments. The somewhat on-the-nose organization makes him a counter-offer, though. They'd like him to get people to sell their souls in exchange for wishes, betting on the short sightedness of mankind versus the proof of the existence of souls. You have to believe in a soul to actually sell it and I think that's a nice twist on matters. Shane is immoral enough to believe he can game the system, stay out of hell, and yet send endless numbers of other people there. He only has a three soul a month quota after all.
Shane is an appalling human being but a believable one because the only people he does care about are his fiance Maggie, his friend Dale, and himself. He has such contempt for Dale's Catholicism that he can't bring himself to respect the idea of good vs. evil when he's dumped in the middle of it. There's a bit of a critique of capitalism going on in the book but no more than any other media which asks what happens when money is more important than people.
The book hurts for one element and that is that we never quite buy the fact Shane isn't a complete scumbag. It's a bit like my problem with Breaking Bad that I was never able to make the disconnect other viewers did that selling meth was the proper answer to Walter White's problems. Shane, upon discovering hell is real and the Devil actually buys people's souls, has no problem willingly taking a job to condemn other people to eternal suffering. It marks him as a monster from the beginning and thus his corruption is not an ongoing process like the author believes but complete midway. That doesn't diminish my enjoyment, though, because I liked the discovery SHANE has that he's an awful person.
Satan's Salesman is a horror novel but it is a horror novel in the Stephen King sense that its' about how a "normal" human being can get caught up in supernatural events simply because he refuses to treat it as any different from any other job. At one point, he gets a good man to sell his soul for the "greater good" while blinding him to the fact he'll be devastating his family both living as well as dead. It doesn't take much of a pitch to get Shane to also feel like he's not REALLY at fault for the horrible consequences of the things he does. There's some Needful Things and Thinner influence in both, I think, and I wouldn't be surprised if Matthew Davenport drew from either.
The ending is great and I think it begs for a sequel or, at least, a continuation of Shane's story as a villain in Broken Nights or a second installment. This is a short book but definitely one worth the money. Infernalism is rarely given a good treatment in books due to how familiar it is to us but here, it works well because it is the banality of evil given just the touch of the diabolic and both made worse.