Eric plays on the Faust legend, and it read to me as a slighter work than the Discworld novels that immediately preceded it in publication. Wikipedia tells me that Eric was originally published in a larger format, fully illustrated by Josh Kirby, who did most of the covers of the UK editions of the Discworld books (until his death in 2001). Reading the story as something closer to a graphic novel might have made it more fun and obscured the lack of heft.
The long and the short of it, though, is that at this point in the series I like Rincewind a lot less than Pratchett does. I gather that he’s supposed to be something of a wizardly everyman, but he strikes me as a blank slate upon which nothing has been written. In this story, he isn’t as magically hapless as he is in other books — for reasons that are explained toward the end of Eric — but that line of comedy played out for me quite some time ago, as did the running (pardon the word) gag about his Luggage being able to find him no matter how far they have become separated in space, or indeed time.
Structurally, this is the Search for Spock of Discworld. At the end of Sourcery, Rincewind wound up in the Dungeon Dimensions, presumably forever. To get him back, Pratchett has Eric, a young demon hacker, trying to summon up someone like Mephistopheles. Eric’s spells produce Rincewind instead. There is bargaining, though no soul is promised, and Eric compels Rincewind to promise to grant him three wishes. Very much to his own surprise, Rincewind can deliver, but as is the way of these things, Eric does not get what he thought he was getting.
By the end, though, it’s all sorted.