So, the interesting thing about this series is that the first person narrator is a sixteen-ish year old sociopath trying to block his murderous tendencies. As a writer, I can tell you that this is crazy because one reason you write a first person story is to make your character more relatable, but the protagonist of this series most certainly does not fit that bill.
If you've seen Dexter or Death Note (though I haven't seen Dexter), I can tell you this series is pretty much just a novel adaptation of those stories. By that, I mean that the main character is a serial killer, but the people that he is killing are "worse" murderers than he is. These guys are only a threat to the worst of the bunch, but at the same time, they are still serial killers by definition.
As a writer, this series has provided a lot of insight as to the ins and outs of dead bodies. John Cleaver works in a mortuary and sees the victims personally, so my understanding of what happens to a body when you die has increased substantially. Hey, you never know what kind of stuff you'll need to know when you're writing. I've researched everything from mending broken limbs to the anatomy of horses for crying out loud. If I'm going to get into the epic fantasy genre, I'll need to know how dead bodies work. For science.
A third interesting thing about this series (minor spoilers, though you're told this somewhat early on), is that the serial killers John is hunting down are 'demons' in a sense. They are not normal people, so they don't abide by the same rules. What police would consider impossible for a serial killer to do could be easily attributed to what a demon can do. Maybe he can fly. Maybe he can turn invisible. And since John is obsessed with serial killers, he has the skill of being able to track them down a lot better than normal people, but at the same time he's still a kid barely in high school trying to find a supernatural murderer.
So, while this isn't my favorite series (I would have changed the endings to the second two books I read), I'd recommend it if you're a reader that enjoyed either Dexter or Death Note, like morbid stuff, or are really just interested in psychology. The problems I have with the series are mainly concerned with how the author decides to move things forward or manipulate his characters. The plot dynamic and the characters are mostly fine, but I feel like if I was experienced as he was I could write it better. Of course that's easy for a wannabe like me to say, though.