I suppose for a beginning we should really consider what detective fiction is. A reasonable definition would probably be that it would be a story based on the investigation of a crime, mostly, but not always a murder. By a detective who in the early days would usually be a gifted amateur of independent means.

Probably, in the broader realm of general crime fiction, detective fiction is the most popular, combining mystery, intrigue, all elements of society, and any physical background you could think of.

As well as independent means, the earliest popular fictional detectives would be somewhat eccentric, and have at least a few character flaws to make them interesting (Sherlock Holmes' drug habits for example). They would often have an assistant (Dr. Watson for Holmes), who would be loyal, staunch, and a little slow on the uptake, although by no means stupid. The assistant would come in handy for talking over theories, and through that character, explaining things to the reader.

It is generally accepted that the first "modern" detective story on these lines was "The Murders In The Rue Morgue" by Edgar Allan Poe, published in 1841. In this short story Poe introduced his detective C. Auguste Dupin. Genius and eccentric, Dupin is generally thought to be the inspiration for Sir Arthur Conan Doyle's Sherlock Holmes. Poe worked to a formula that has been used, with a few changes here and there, ever since. The hero has to solve whatever mystery is presented by using logic, observation, and occasional flashes of intuition.

The authorities are usually described as inefficient bunglers, who after a few false trails, often involving accusing the wrong person, grudgingly accept the help (interference) of the lead charcter. Who magnanimously solves the case, and lets the authorities take the credit. Although the main characters and the reader are left in no doubt as to who is really responsible. On occasion the detective will accept a fee from a wealthy client, but will more usually turn it down with just a touch of arrogance.

Sometimes in the early days of detective fiction authors would use real life events as inspiration for a story. One such, was the infamous case of the murder of Mary Cecilia Rogers in New Jersey in 1841. It was a brutal murder, her body found floating in the Hudson River, having been subjected to considerable violence. No-one was ever brought to account for her death. Edgar Allen Poe used this story for the basis of his second novel featuring C. Auguste Dupin. He changed Mary Rogers name to Marie Roget, transporetd the whole thing to Paris and had Dupin solve the case.

From these beginnings grew a whole genre, which we still enjoy, possibly even more so, today.