I like this the best of all Beagle's books.

To borrow from a review:
"Anyone who has met the Society for Creative Anachronism will instantly recognize the behavior they encounter in this story; against the backdrop of medieval jousting tournament reenactments we meet a goddess, a man who can channel a Viking, and a talented young witch who gets dangerously involved with an evil spirit."

From the publisher: "Folk stars our good friend Joe Farrell (and his ancient Volkswagon van, Madame Schumann-Heink), last seen in Lila the Werewolf and later to be featured in Julie's Unicorn. All is not as it seems at the local West Coast medieval times reenactment club, or maybe it's too much what it seems. People dressing up as witches are suddenly casting spells and summoning really bad things from elsewhere, people dressed in chain mail suddenly have really bad teeth and speak kind of Middle-Ages-like. What's a slacker to do? Well, if you're Joe Farrell, you find an old girlfriend, cook a lot of really good food, and generally find yourself in the middle of Really Big Things."

See also The Last Unicorn.