It was T S Eliot who described Wilkie Collins' "The Moonstone" as "the first, the longest, and the best of Modern English detective novels". Not everybody might agree with this, but all practitioners, readers, and fans of detective fiction will find much to admire and enjoy in this magnificent 1868 publication.
Although not exactly the first example of detection novels, it provides the archetypal sleuth, Sergeant Cuff, an astute though idiosyncratic detective who leads the chase to the solution of the mystery, easily surpassing the dim-witted local police authorities. It also explores the full potential of the whodunit formula.
Arguably, it is still the longest example of detective fiction. Unlike most other serialized novels of its era, this one is meticulously plotted. You'll find red herrings, suspense, the unexpected, climaxes that overwhelm or fizzle out, and a satisfying denouement. It is narrated largely by some of the principal characters. All are revealed in well-rounded perspective while carrying forward the story line. The most popular has always been Drusilla Clack, "that rampant spinster", a self-righteous tract-dispensing lady who likes to eavesdrop and to be judgmental.
Is it the best? I would unhesitatingly award it the prize, while welcoming other internet browsers to name other contenders.
Wealthy internet browsers are recommended to download the unabridged audio reading of the book. It is a novel that reads well, and the full length reading available is a model of its kind. Naxos has produced an abridged version. It has the benefit of multiple readers, but most of the charm and all the atmosphere seems to disappear in the abridgment process. Book format will put you in touch with the original text and, provided you have the leisure and disposition for tackling a 20 hour read, will provide your imagination, your mind and your literary appetite with rich material.