The Hitchhiker's Guide (to the Galaxy) trilogy, sometimes called HHGG, was written by the late Douglas Adams. It consists of five books.
The first book in the HHGG series was The Hitchhiker's Guide to the Galaxy, and the name of the Gatehopper's Guide to Proelium was an intentional reference/homage to the series. The 'Guide' in the books is an electronic encyclopedia, which has the words DON'T PANIC printed on its cover in large, friendly letters.
HHGG is not the same as [h2g2], an online community and collaborative encyclopedia based on the Hitchhiker's Guide. h2g2 was Douglas Adams's idea.