H.P. Lovecraft Holiday Fun Club
This was the rather enigmatic subject of the dedication of an earlier Discworld novel, Equal Rites. Research has discovered that this is Pterry's reference to a group of what were then new wave British fantasy and sci-fi authors who seemed to attend just about every conference and fan-meet in the late 1980s and early 1990s. These included gothic conventions at Whitby - in fiction, the point where Dracula made his landfall in England. These were generally hosted at a nearby holiday camp of the Butlins/Pontins variety.
Mary Gentle's unique take on fantasy fiction is believed to have influenced Thief of Time and was a very definite influence on Unseen Academicals, where Terry made at least one very deliberate homage to her novel of Orcdom, Grunts. (see annotations to Unseen Academicals)
A third Happy Camper is fantasy/SF writer Kim Newman (also refer to Reading Suggestions) whose take on the Dracula mythos involves Drac doing some inventive thinking on what might happen if a ruling monarch invites a vampire into her home - and by extension Kingdom and Empire.
Members of the Club are also namechecked in the Dedication to Guards! Guards!. A Mike Harrison is also namechecked in this dedication alongside Gentle and Gaiman. While it has been suggested that this is Mike Harrison, lead singer of the interestingly named band Spooky Tooth, it is overwhelmingly likely that this is the New Wave science fiction writer M. (Michael) John Harrison, a near-contemporary of Pterry and a dominant figure on the scene that he is certain to have encountered.