This seems a bit of a change of pace from Liz Williams's well-received earlier novels, such as The Poison Master and Banner of Souls. The tone is a bit lighter, the setup simply more playful -- which isn't to say that the book is comic or without darker moments. And it isn't to say that Snake Agent isn't enjoyable -- indeed, it is a fine, clever, absorbing novel.
It is subtitled "A Zhu Irzh & Inspector Chen Novel", suggesting the possibility of a series. And I have seen a couple of short stories set in the same milieu, including one ("Adventures in the Ghost Trade", Interzone, April 2000) from which the current novel is expanded. The setting is a curious combination of near future SF with Chinese-based fantasy. It might be called an "Alternate History Fantasy set in the Future", by which I mean that it takes for granted a world much resembling ours in which certain fantastical aspects are true, extrapolated into the near future. The location is a new city called Singapore Three, somewhere in South China. Inspector Chen is a policeman assigned to cases involving Hell -- cases where demons have intruded into our world, or the dead have ended up in the wrong place, or humans are engaged in illegal transactions with the underworld. He is uniquely suited to this duty: he has been granted the protection of a goddess, Kuan Yin, for his occasional trips to Hell, and he also has a domestic secret: his wife is a demon.
The story opens with an investigation into a recently deceased young woman, the daughter of a rich man, who seems to have gone to Hell by mistake. Or so her mother says. But before long Chen's investigation bumps into some political roadblocks. It seems highly place interests, in both worlds, are involved in something unsavory. And indeed, down in Hell, Seneschal Zhu Irzh is investigating a case of the souls of virtuous young human girls being taken to Hell for use as prostitutes. Before long, he and Chen meet and realize that their cases are connected. But Irzh, too, finds that political interests in Hell are impeding his investigation. And, finally, Inspector Chen's wife, Inari, is being chased, presumably by the vindictive family of the demon she had refused to marry before escaping to Earth.
This central mystery develops quite satisfyingly, leading to a resolution that is effective enough, if perhaps just slightly too rapid and too pat. Both Inspector Chen and his infernal counterpart, Irzh, are engaging characters, reflecting their different origins nicely. Irzh in particular is an amusing creation, for the most part a conventional demon but with urges toward compassion and other virtues that he tries to suppress. Williams's description of Hell is also fun and nicely cynical -- especially its rival Ministries: Wealth, War, Epidemics, etc.
This isn't a major work, but it is an enjoyable novel with interesting protagonists. I would be delighted to see further stories about the same pair of detectives: I think the setup quite fruitful, and the lead pair interesting enough to follow for some time.