I am going into this review assuming that you, unnamed reader, have also read I Am Not A Serial Killer seeing as Mr. Monster is its sequel. If you have not and do not wish to be spoiled on the preceding book's plot events, consider this your warning. 
 Mr. Monster continues the story of John Wayne Cleaver, a 16 year old high school student who also happens to be a diagnosed sociopath. Obsessed with serial killers, John uses over abundance of morbid knowledge to set up rules for himself to prevent him from also becoming a serial killer. These rules were broken last summer when people started showing up dead around quiet little Clayton county, dead and missing miscellaneous body parts. But now his dark side, aptly named Mr. Monster, is no longer caged behind the rules John broke to kill the demon terrorizing his town snd a new killer has begun its own killing spree in Clayton. 
 Mr. Monster explores what it is like to have a sociopathic mind with much more depth than I Am Not A Serial Killer did. Because John has now gotten a taste of killing and broke all of his rules in order to stop demonic Mr. Crowley (sidebar, but I'm annoyed at myself for NOT guessing his demonic background earlier in the preceding book. His name is CROWLEY! HOW DID I MISS THIS. I am ashamed.) the line between John and Mr. Monster is much fainter, a lot more blurry, and since most readers will not be sociopaths, we notice his dangerous nature far before John does. I loved this aspect of the book. There's a particular scene where John is getting ready to go out with other people and he's convincing himself that is it perfectly logical to bring a knife with him because, "you never know! We might go fishing! I'd NEED it then! Oh, it's perfectly fine that I am semi-stalking the girl I like because I need to know what kind of things she likes so I don't seem weird when we talk! This is normal!" Meanwhile, the reader recognizes that this is just Mr. Monster taking over, clouding his thoughts so he doesn't realize he's breaking his rules he set up to protect others. It was handled really nicely and made it clear that although John calls his darker side "Mr. Monster" he does not have split personality disorder or anything similar. It is easier to assign a separate identity to his desires to torture and kill people, but it really is just part of him. John is the one wanting to do these things but he also hates that part of him. When he realizes it is in control, of course. 
 The war between John and his murderous tendencies is really what drove the plot of the book for me. The actual plot in and of itself is a tad disjointed. Whereas I Am Not A Serial Killer was focused on John systematically breaking his rules to stop the demon, culminating in John's abuse towards Mrs. Crowley in order to draw out her husband and kill him, Mr. Monster seems fairly split in two parts: John's problems with his personal life, be it with Brooke or family conflict, and John discovering who the killer is and being kidnapped by him. Maybe it was meant to be a reflection of John's own dual nature, but the two plots still felt disconnected from each other to me. 
 Thankfully, even though the two plot threads are fairly separated for the majority of the book, they are both quite powerful on their own and when they DO eventually merge, the resulting conflict is handled marvelously, allowing for some very tense moments when the reader doesn't know what John is going to do. The final climax felt incredibly tense and the loss felt in its aftermath was honest and real. As someone who frequently watches crime procedurals focusing on violent and dangerous killers, it can be a really fine line to walk when trying to create a satisfying ending that also has significant loss, but Mr. Wells handled the ending very well. 
 My criticisms that existed with I Am Not A Serial Killer still stand for Mr. Monster: the aftermath of the climatic confrontation is too rushed, I wanted more time to react with John and his family and conflicts with previous characters are implied to be resolved but we don't get to fully see it. However, although this was a minor plot point in terms of the overall story, there was one thing that I kept coming back to in frustration: John's mom never made him see a new therapist. John's mom who was so adamant and worried about her son's potential for becoming a killer in the last book doesn't make him start seeing someone else now that his previous therapist was murdered. And it's not like his mom changed character between books. She still is very worried about her son's disorder and is scared of what he'll become. She forces him each morning to recite his rules in order to try to help rebuild those walls he broke down. And she SAW the trauma he went through when fighting the demon. I cannot believe that she wouldn't keep pushing and bothering him about seeing a new therapist. John is still visibly struggling with his homicidal tendencies and the personality abscence left in the wake of the previous (I'm forgetting his name right now, I apologize) therapist's death is strongly felt. 
 Mr. Monster took great strides forward as a sequel, building upon the foundations established in I Am Not a Serial Killer and adding new layers to the mythos. Also! The audiobook for Mr. Monster got a new narrator who is MUCH better than his predecessor. His vocal range is of a higher caliber and he was a delight to listen to. At the end of the book, John makes a phone call to someone spoilery but the set up for the next novel literally made me grin with eagerness. I am very excited to see what will happen next to John and the city of Clayton, although I fear for the safety of the characters we've grown to love over the course of these two books.