The Murder of Roger Ackroyd can be classified as a detective novel, which is a kind of mystery novel. Like other mystery novels, this one centers on a single, main mystery that drives the plot forward. The investigation of the main mystery gives rise to other, smaller mysteries. For instance, Poirot finds a wedding ring of unknown origin that turns out to belong to Ursula Bourne (now Ursula Paton). This secret marriage is not directly relevant to the murder of Roger Ackroyd, but solving it helps Poirot put all the pieces together to identify Dr. Sheppard as the murderer. This structure (a main mystery with intertwined, smaller mysteries on the side) is common in mystery novels in part because teasing out all the relevant details from the irrelevant ones makes for a compelling puzzle. Agatha Christie is especially known for her use of red herrings (irrelevant details that distract) and Chekhov's guns (important details that will prove indispensable to the main plot).
Christie's novel has more than the mystery novel's characteristic twisting and turning plot. Its focus on Hercule Poirot, the eccentric Belgian detective who solves the case of Roger Ackroyd's murder, makes it a detective novel. Detective novels are invested not only in the solution to the central mystery, but also in the experience and process of the person tasked with solving it. Poirot, who features in many Christie novels, remains one of the most famous fictional detectives. He represents an idealized kind of detective work that police typically do not have the resources or incentive to perform. Poirot takes a great deal of time and care in interviewing suspects and collecting evidence. He is also extremely smart and specializes in deductive reasoning. Whereas the police are often compelled to close cases quickly and therefore rush their investigations, Poirot is the kind of detective people who think of themselves as law-abiding and ethical citizens would like to have on a case because he always finds a way to find the truth and identify the guilty party.
Christie's novel is part of a larger genre of detective fiction, including Sir Arthur Conan Doyle's Sherlock Holmes stories, that perpetuates the fantasy that there is always a way to uncover the whole truth behind a crime. The victims in these books are often, like Ackroyd, wealthy and white. While Christie does play with the idea that anyone can be a killer, there is still a simplistic sense that crime always has clearly traceable, sinister motives and that every "criminal" leaves enough evidence behind that a smart detective can determine exactly what happened. These stories are satisfying to read, but it is important to note the ways in which they have impacted real-world systems of justice. For example, false convictions due to shoddy "expert testimony" have come under public scrutiny in recent years. Often, the trust juries place in so-called "experts" is informed by the idea that a deductive process like Poirot's is foolproof.
