Can anyone identify this book?

I can’t remember the title or the author. It was a sort of fantasy story, about a man who is a composer, and is working on writing an opera about Captain Nemo. He visits a town he’d been to before, I think it was on the north-east US coast somewhere. The last time he was there on vacation his wife had fallen from the cliff into the sea and died.

This time, weird things are happenning and the towns-people seem to be divided against each other. The composer starts to investigate and discovers that the town has a secret that some of the residents are sworn to protect.

Halfway through the story he is joined by someone (can’t remember if it was his brother, or a colleague) but he’s dressed up as Captain Nemo and trying to help the composer finish the opera by really getting into the character as a sort of inspiration.

Weird things continue to happen and the composer is brought into the secret of the town, which is that there is a vast underground cave, and some people in the town intend to live down there permanently. The Captain Nemo guy turns out to be a nutcase and is controlling the townspeople, and kind of brainwashing them into wanting to live underground. In the end, the composer-guy rescues them.

I think the title might include the town name but I can’t remember what that was. I think it began with E. I’ve tried googling this without success. Does anyone recognise it?