The relationship itself is almost unexplored

I recently completed The Corpse on the Dike by Janwillem van de Wetering.

Van de Wetering was a man of experiences and wanderlust. From Wikipedia

Van de Wetering was born and raised in Rotterdam, but in later years he lived in South Africa, Japan, London, Colombia, Peru, Australia, Amsterdam and most recently in Surry, Maine, the setting of two of his Grijpstra and de Gier novels and his children's series about the porcupine Hugh Pine.

Van de Wetering studied Zen under the guidance of Oda Sessō, together with Walter Nowick, at Daitoku-ji. Van de Wetering lived a year in Daitoku-Ji and half a year with Nowick and described these in The Empty Mirror. Van de Wetering describes a visit to the monastery by the highly respected Hugo Enomiya-Lassalle, describing his own mixed thoughts about this representative of what he deemed an old-fashioned religion. Sōkō Morinaga, Walter Nowick's Dharma brother, wrote in Novice to Master about traditional practices at that time.

Toleration leads to friendship. Friendship always wins. There has never been a Buddhist war
— The Empty Mirror: Experiences in a Japanese Zen Monastery (pg 74)

His many travels and his experiences in a Zen Buddhist monastery and as a member of the Amsterdam Special Constabulary "being a policeman in one's spare time" as he phrased it in his introduction to Outsider in Amsterdam) lent authenticity to his works of fiction and nonfiction.

Also from Wikipedia.

Janwillem van de Wetering was particularly noted for his detective fiction, his most popular creations being Grijpstra and de Gier, a pair of Amsterdam police officers who figure in a lengthy series of novels and short stories. Most of the mysteries are rich with images from Amsterdam, where most of them take place; some also feature a cat: in earlier novels, one named Oliver; in later novels, a female named Tabriz. He also wrote stories for children and nonfiction works. He usually wrote in Dutch and then in English; the two versions often differ considerably.

His two protagonists are Adjutant-Detective Henk Grijpstra and Detective-Sergeant Rinus de Gier. Grijpstra is middle-aged with a family life and somewhat disappointing marriage. de Gier is younger, a ladies man, almost a dandy but a thinker and an amateur flautist.

The distinctive feature of the series, other than the flavor and insights about Amsterdam in which it is set, is the distinct working relationship between the partners. The relationship itself is almost unexplored. We know something of the one and something of the other and a little about how they work together but not a lot. It is a technique that to some degree allows you to see and interpret events in the plot from two different vantage points at the same time. An unusual device.

But van de Wettering makes it work effectively. They tend to be quiet steady books but I find them enjoyable.