L: Change the WorLd is a novel by an author who uses the pen name "M." It is the novel adaptation of the third movie in the live action movie series Death Note, which is based off of the manga by Tsugumi Ohba and Takeshi Obata. It will make a lot more sense if you have prior knowledge of the series (specifically of the movies, as they are notably different from the manga), though is still enjoyable if you haven't. The story is about the world's greatest detective, L, who must stop a terrorist organization called "Blue Ship" before his inevitable death in twenty-three days. The main theme is that no human has the right to decide who deserves to live and who deserves to die.
I'm a huge fan of Death Note myself, and pretty much everyone in the fandom has gone through some huge phase where they were completely obsessed with L. To be honest, I'm far past that point, and L in this novel isn't exactly true to the original from the manga. I'm not going to say that none of the characters interested me, though; almost all of the Blue Ship members (I didn't feel very strongly about Yoshizawa or Kujo) caught my attention pretty quickly. As a whole, they get a lot more elaboration on their characters as a whole than they did in the movie, which is part of the reason I like Konishi as much as I do.
L:CtW is really a novel that is easy to like, but also easy to hate, depending on what kind of book you like. As I mentioned earlier, L isn't exactly like his canon self, so if you'd rather stick to canon L, you might have a problem with his portrayal. If you just generally like L a lot and don't care too much about a bit of change from main canon, you'll probably enjoy it. As for the story outside of L, it's alright; honestly, it's not nearly as good as the Los Angeles BB Murder Cases novel (or the rest of DN, for that matter), but it's still okay if you're not comparing it to the original.