There are 3 books in The Gatekeeper Trilogy series.
The Gatekeeper Trilogy series does not have a new book coming out soon. The latest book, Sons of Entropy (Book 3), was published in April 1999.
The first book in The Gatekeeper Trilogy series, Out of the Madhouse, was published in January 1999.
The Gatekeeper Trilogy series primarily falls into the Urban Fantasy genre. This young adult series was written for ages 12-17, but can be enjoyed by adults too.