I'm not sure specifically as I haven't read it. But generally, in the context of 'isekai' (another world) type novels, it might be about a hero who returns from another world and becomes the strongest in the modern world.
I'm not familiar with the 'makai kaeri no rettou nouryokusha light novel'. However, light novels typically blend elements such as fantasy, adventure, and sometimes romance. It might be about a hero's journey in a mysterious world, or it could center around a group of characters with special powers trying to achieve a certain goal in a unique setting. Without reading it, it's hard to say for sure.
You can try looking for it in local bookstores that specialize in Japanese literature or light novels. They might have it in stock or be able to order it for you.
Overall, the similarities outweigh the differences. The anime mostly adheres to the plot and characters of the manga. Some minor changes might be in the animation style or added background details for better visualization.
To draw a cute elephant outline, begin by making a round head and a slightly elongated body. Sketch the trunk coming out from the middle of the face. Add cute, big eyes and floppy ears. Make the legs short and chubby for that adorable touch.