From the introduction:
There's trouble in the Island Kingdoms. Elfin princesses are being kidnapped, troll gatekeepers are not showing up for work, and shopkeepers are making customers promise to return the items they get from the shops! Everything is topsy-turvy, the fair folk are flustered, and there's only one explanation - there are bogies about.
Bogies - those ballybogs and bogarts, bug-a-boos and bogeymen, boggans, hobbers, gobs and gobelins. Call them whatever you like - it all means the same thing - trouble.