Dirk Gently's Holistic Detective Agency is a series of books written by Douglas Adams.
It shares two noticeable similarities to the Hitchhiker's Universe. In the first book, Dirk Gently hands Richard Macduff a towel. Richard asks, "Do you carry a towel with you everywhere?" This is an obvious reference to the Guide's stance on towels.
The other is the inclusion of the character Thor, which both series employ as a character. This could mean the two series exist within the same Universe.