A Line to Kill by Anthony Horowitz
The hook here is that Horowitz has written himself into the book, a Watson figure to Hawthorne's Sherlock. It seems a silly conceit to me. I guess it lets him mention his other work, but we all know this is a fictionalized version of Horowitz, basically a character, so I don't see the point.
Anyway, this time around Horowitz and Hawthorne are sent to the island of Alderney for a small weekend literary festival. And of course, while they're there, someone is killed - a wealthy sponsor of the festival, murdered at his own house party. The island is locked down, no one allowed on, no one allowed off, while the police, with Hawthorne's help, try to figure out who the killer is. Everyone on the island seems to have a reason to want the man dead.
The house party/isolated island gives us a limited number of suspects, but everyone here has a secret and there are red herrings galore. Horowitz...