Eric wants to summon a demon to make it grant him three wishes. The current ruler of Hell, Astfgl, is quite keen on the idea, because he figures the demon he sends can then bend the boy and his skills to their advantage. But Astfgl has also been quite busy changing how things are done in Hell, away from conventional tortures, towards actual soul-crushing approaches. Which has caused a fair amount of resentment among the former elite. So they instead arrange for a cowardly wizard, Rincewind, who was trapped outside his home dimension to be sent instead, and insure that things do not go as Astfgl plans.
I've never read any of the Discworld books prior to this, but Pratchett's quite good at making sure whatever backstory or rules of the characters and universe the reader needs are provided. I never felt lost while I was reading it. A little confused about what the deal was with Luggage, mostly in terms of why it was so mean, but I understood why it would turn up where it did. The writing tells you what you need to know about the characters through their dialogue and actions. It was clear this wasn't the first adventure Rincewind had been drawn into, and I'm guessing it wasn't his last, but it works well as a standalone story of its own.
There's quite a lot of humor in it. Pratchett's good at setting things going one way, then providing a humorous twist. Or sometimes it's simply a matter of looking at something common from a different angle then you might expect, and going with that. He does a play on the Trojan War, but offers up certain commentary on the tactical skills (and casualty rates) of brave commanders versus cowardly ones. I'm not sure I totally agree with the conclusion - I think there have been plenty of commanders who were cowards but still had high casualty rates, because they weren't anywhere near the actual fighting they were commanding - but he writes it well.
I had a sense reading the book there was something in there about the difficulty of affecting change. How one can try to move things in a new direction, but the inertia and resistance will drag things back to their prior state. maybe it's about the nature of humans to resist changing themselves. Rincewind believes running away is the best option in most any situation, and he staunchly resists any circumstance that tries to make him behave different. The Luggage follows him, and won't be stopped by anything, even the end of existence. Eric would probably be the acid test. Whether this whole experience had any impact on his life's trajectory.
'Rincewind sighed. He'd tried to make his basic philosophy clear time and again, and people never got the message.
"Don't you worry about to," he said. "In my experience that always takes care of itself. The important word is away."'