There are 2 books in the Derkholm series. The newest book is Year of the Griffin which came out in 2001.

1
Dark Lord of Derkholm

Book cover of Dark Lord of Derkholm
Everyone - wizards, soldiers, farmers, elves, dragons, kings and queens alike - is fed up with Mr Chesney's Pilgrim Parties: groups of tourists from the next-door world who descend en masse every year to take the Grand Tour. What they expect are all the trappings of a grand fantasy adventure,…
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2
Year of the Griffin

Book cover of Year of the Griffin

It is eight years after the tours from offworld have stopped. High Chancellor Querida has retired, leaving Wizard Corkoran in charge of the Wizards' University. Although Wizard Corkoran's obsession is to be the first man on the moon, and most of his time is devoted to this project, he decides…