A bit of a sidestep for the Discworld series, but funny nonetheless. 
 This novel has the feeling of nice little sidestep from the main series. There is plenty of humor and even some self referential mockery of the series itself. In short Eric a wouldbe Demonologist tries to summon a Demon, but instead summons Rinswind from the dungeon dimension where he was traped (see Sourcery for reference). As you'll understand by now things go from bad to worse from then on. 
 In summation: This novel has a bit of an interlude feel to it, but keeps all its comedic and ironic humor directed at its subject. In this case the discworld itself as series.