There are 4 books in the Investigators of the Unknown series.
The Investigators of the Unknown series does not have a new book coming out soon. The latest book, Angela's Aliens (Book 4), was published in September 1996.
The first book in the Investigators of the Unknown series, The Gold Dust Letters, was published in April 1994.
The Investigators of the Unknown 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.