The Ghost Twins series does not have a new book coming out soon. The latest book, The Mystery on Walrus Mountain (Book 3), was published in January 1995.
The first book in the Ghost Twins series, The Mystery at Kickingbird Lake, was published in September 1994.
The Ghost Twins series primarily falls into the General Fiction genre. This middle grade series was written for ages 8-12, but can be enjoyed by older readers as well.