It's an historical novel set in the Dutch trading post in Nagasaki at the start of the 19th century. Japan was otherwise closed to foreigners, and knowledge of European languages and religion was banned. De Zoet is a meek but stubborn clerk, marooned among scoundrels. The other main characters are a Samurai midwife called Miss Aibagawa, who he falls in love with, and a Japanese translator, who also loves her. All three are in liminal positions, all wounded.
The first part of the story, mostly told from Jacob's point of view is fairly conventional historical fiction, though it's good fun. The middle section, told from the view of the two Japanese characters is much more interesting. Miss Aibagawa is abducted into an ancient cult which - well, I won't say - but there is a powerful and gradually unfolded evil. The writing here is brilliant. Japan is not westernised, not idealised, not made alien, just brought to vivid life.
Throughout we experience the varying religious beliefs of the different characters, and it seems almost as if the Universe responds to those beliefs. So, prayers are answered, and Shinto rituals are (arguably) effective, and supernatural forces arrange for strange coincidences to happen. And these coincidences increase in significance as the plot progresses. As a result you might find the last section a little too much to stomach. I didn't; I thought the characterisation and the writing were strong enough to make it work. It's no more absurd than Dickens or Austen, say. And all somehow enfolded by the suggestion that the people are courageous tools of a hidden benign but ruthless force.
ETA an old, old, friend who I had lost touch with just got in touch this evening and blow me down, he'd just read this novel too. He said that he thought the benign force was perhaps the Dao.