The Cassie Hartt series does not have a new book coming out soon. The latest book, Mystery of the Island Jewels (Book 2), was published in June 2002.
The first book in the Cassie Hartt series, Mystery At Kittiwake Bay, was published in December 2001.
The Cassie Hartt series primarily falls into the Mystery genre. This middle grade series was written for ages 8-12, but can be enjoyed by older readers as well.