|This isn't the edition I own, but I couldn't resist the image.|
The detective story was already well established by the time the first Holmes novel appeared in 1887, of course. Edgar Allen Poe had invented it in the 1840s with his three C. Auguste Dupin short stories. Wilkie Collins, Emile Gaboriau, and Anna Katherine Green expanded the genre to the novel form in England, France and the United States respectively.
Without Dupin, as Conan Doyle readily acknowledged, there would have never been a Sherlock Holmes. But without Sherlock Holmes, the detective story may never have become the almost universally appreciated genre it is today. Conan Doyle took Poe's formula, improved it by almost always playing fair with the reader, and popularized it to a degree that Poe could scarcely have imagined. He achieved an enviable commercial and critical success, which inevitably inspired imitators and successors in the field.
Even Sam Spade, Philip Marlowe, and their ilk in the hard-boiled school of private eye fiction -- often assumed to stand in contrast to Holmes, actually share much with him. Steven Doyle, in Sherlock Holmes for Dummies, makes a convincing argument that the last half of the final Holmes novel, The Valley of Fear, "is the world's first hard-boiled detective story."
Haycraft summed it up well in Murder for Pleasure: "But for the tales in which he appeared, the detective story as we know today it might never have developed -- or only in a vastly different and certainly less pleasurable form."