Originally, I had not bothered to read these since the events were covered (in abridged fashion) by the first Anime adaptation. After recently watching the second season though I came to realize that the adaptation didn’t merely skip events. It actually goes and changes some things as well… quite illogically at times in that season’s case. As a result I decided to read them after all.
The first novel is covered by the first five episodes of the TV series and unsurprisingly ends up not actually having much skipped content beyond the students’ introduction to the world (which was eventually covered by a special episode). The second novel then gets compressed into the sixth and seventh episodes, and yet… you still don’t really miss much. Nothing all that important was actually cut out, and the Labyrinth exploration in particular was edited extremely well.
The third switches things up by having been adapted into episodes eight through ten, yet again with nothing of particular value being cut out. Finally we come to the fourth novel which similar to the last one was also adapted into 3 episodes. A few more noticeable things were changed here than for the previous novels, yet in all cases they merely serve to streamline events. The only real absence that hurts is not seeing the hero party progress until they’re already near the end.
So, all-in-all, if it weren’t for the dire state of the first Anime’s action scenes it would actually be a pretty damn good adaptation and can easily be watched in place of reading the novels (assuming you have a tolerance for periodically abysmal animation quality). Frankly, I’m shocked at how well whomever was in charge of the series composition managed to smooth out the event progression.