Was Harper's Island based on a book?If yes, kindly tell me the titre

Not that I know of.
But the writers seemed to be heavily influenced par Agatha Christie's "And then there were none". Also called "Ten Little Indians".
Which is one of the most unique mysteries she wrote. The suspects get killed one par one and, like most her novels, the killer is revealed in the end.

The script of the pilot has 2-3 descriptive references on Agatha's ATTWN novel.

It Was Based on a fairytale story that the producers haven't donné much info on but they did tell us that one of the producers heard it when he was a kid and then decided to make it a tv show.

well it wasnt directly based off of a book but the producers a dit an inspiration was the book "then there were none" par agatha christie so no offical book but there were inspirations