Christopher is part of a long literary tradition of first-person narrators with unreliable or otherwise compromised ways of telling their story. Although Haddon's novel may seem new and unique in structure, it is also a modern example of a 20th century literary form.
Christopher is what was once known as an idiot-savant. He has difficulty with things that should be easy, like reading emotions, and finds easy things that should be difficult, like remembering exact details. It is a not uncommon psychological phenomenon that people with autism spectrum disorders and other developmental and cognitive disabilities will show extreme proficiency in one or more isolated skills. Christopher manifests a good deal of these savant characteristics, and his autism is mild enough to allow him to be extremely high functioning - these two characteristics combine to make him an excellent narrator.
He is not dissimilar in this way from his hero, Sherlock Holmes. Although Holmes is not a narrator (his stories are narrated by his colleague Dr. Watson), he is also extraordinarily gifted at isolated skills - and has a very hard time understanding emotions and interacting with people in a normal way. In the case of Holmes, the character is so brilliant that he can overcome his limitations through excellent acting and deduction of patterns, but one can see why Christopher so identifies with his fictional hero.
But Christopher has several other more direct antecedents. One of the most famous idiot-savants is Raymond Babbitt in the film "Rainman," as portrayed by Dustin Hoffman. Raymond, like Christopher, is high-functioning autistic with similar savant characteristics, such as performing complex calculations in his head. The film was responsible for bringing autism to greater attention in American culture. It is understandable while Raymond and Christopher are both more sympathetic to normally-abled audiences and readers because of their savant characteristics, but as a result many people have skewed expectations of people with autism spectrum disorders.
Another important antecedent is Charlie in Flowers for Algernon. This 1966 novel had the same gimmick as Curious Incident - it was being written in real time by its narrator, in this case a severely developmentally disabled man named Charlie. But Charlie was undergoing a radical (and fictional) treatment which drastically increased his IQ, and as his journals progress he becomes more and more intelligent. With Charlie, the author Daniel Keyes was able to show us the unfamiliar perspectives of individuals at both extremes of the IQ bell curve, all in the same character.
But some disabled narrators are successful even without characteristics of brilliance. In Faulkner's The Sound and the Fury, the severely developmentally disabled Benjy Compton narrates a quarter of the novel. The prose in this section is extremely difficult to penetrate, due to its stream-of-consciousness style and the difficulty of identifying with Benjy. But it has been extremely influential in modern literary fiction.
On the flip side, there are many brilliant narrators whose disability is not cognitive or developmental, but physical. Helen Keller was one of the most remarkable minds of the 20th century, but from the age of 19 months was completely deafblind. Her books, such as The Story of My Life, give the reader an introduction to how a deafblind person perceives the world, much as Christopher and Charlie introduce us to their perception of the world, but with the lucidity of a brilliant writer.