I read this as an English-language paperback book, bought from a discount bookstore in an Australian (likely Melbourne) shopping mall late 2019. No idea how old it was at the time. The title was bland and "literary" (as opposed to cool and spec-fic-y, the main other category of title according to my made-up rules). Unfortunetely, I have since lost track of said book.
The genre was mystery, set firmly in a city of our world. If I had to hazard a more specific guess I would say a European one. The inciting incident was when our main character (who I'm nearly certain was male) stumbles into a curious case which seems like a standard recluse (also probably male) overdosing, except that the drug in question was unusual. It was a sort of fancy medical opioid, not something that would be easy or normal to find on the street.
Investigation follows, our main character chasing leads etc. etc. though I can't remember this middle bit very well at all. He gets frustrated because while there are some interesting bits of evidence, he can't see any way to tie them together into a conclusive explanation.
At one point, close to the end I think, our main character goes to a dinner party. There's a rich guy (definitely male) who was suspected at some point, though I can't remember if he had been discarded as a person of interest. We have a scene with dramatic Revelations. Either at this point, or shortly after, our main character pieces the story together. The answer to the mystery is one of the things I remember best:
That dead guy was related to the rich guy somehow (bastard son/brother, I think) and was an embarrassment to the family. Therefore, he had to be hidden away. He was provided with a small apartment for himself, which he decorated with tiny figurines, though not much else. These figurines had been seen by the main character when he searched the apartment during the investigation scenes, who remarked on them - they contributed to the oddity somehow, though I can't remember why.
Anyways, to keep the bastard hidden, he was additionally given an endless supply of drugs to occupy his time. The rich guy varied the drugs sometimes, just for kicks. That's how the fancy medical drug made its way into the dead guy's body. Eventually, the rich guy has the dead guy killed; he justifies this at one point by saying it wasn't much of a life to be living.
I am nearly certain at least one of the dead guy, rich guy, or protag was homosexual. If they weren't, then a side character probably was. My brain has this book firmly shelved in its "secretly gay novels" section, though it refuses to explain why.
What novel was this?