This has been on my list for quite some time. I heard the protagonist was autistic and, while it was never explicitly stated, I appreciated this. Characters with autism are hard to come by, and those that are well written are even rarer. I found myself completely understanding him in many ways, even though his autism doesn't manifest in the same way as my autism.
The unconventional chapter markings were quite good. It really helped solidify the idea that the credited author did not write it, considering the main character had a special interest in math and prime numbers. It helped lift him off the page and into my living room.
I liked that the mysteries in the book were only solved halfway through the novel. Too often mystery novels end at the resolution of the mystery, so we don't get a good picture of the fallout that often results from the revelation. Getting to see Christopher, the main character, react to these revelations was wonderful, and honestly very logical, in my opinion.
Overall, it was just what I needed after reading a heavy book in the form of Flowers for Algernon. I feel much better after reading it.