This book featured a main character that lives in a world where some flying machines are used in day-to-day life. However, most people are not active during the night, with the exception of some guild for whatever reason. There is a cult that bans music. The main character is investigating the cult and ends up meeting the cult leader. There is a scene where the protagonist and the cult leader are dining together and, when they discuss the cult, the leader has all of the other followers executed for having heard too much. I'm pretty sure that the cult leader is either a human with some supernatural abilities or is not actually human, but looks like one. Any ideas as to what the book is?
Enough of the details match Tim Powers' "Dinner at Deviant's Palace" to make me suggest it.
This post-apocalyptic novel features a hero, Gregorio Rivas, who is hired to retrieve (and de-program) a woman who has joined a cult (the Jaybushes).
Gregorio is a musician, and the JayBushes do ban music.
At the end of the novel Gregorio has the titular dinner with the leader of the cult, Norton Jaybush. Norton (who was taken over by an alien mind parasite long before) is trying to tempt Gregorio to join him. But Gregorio has grown enough during the novel to refuse. Among other things Gregorio has found that some rhythms will cause the parasite to lose consciousness (which is why it has banned music). On revealing this and other secrets Norton declares that all of his listening followers will have to die. They are sufficiently brain washed that they do not mind much about it.
So we have:
- Cult and cult leader: yes
- Ban Music: yes
- Cult leader is not human, but looks human: yes
- Novel ends with dinner between protagonist and cult leader: yes
- Cult leader intends to execute all listeners for heaving heard too much: yes
But I don't recall anything about flying machines, nor anything about people being unusually inactive at night, or a guild.