I recommend these from a variety of styles:
*The Murder of Roger Acroyd *by Agatha Christie (*Death on the Nile *is also very good IMO)
*The Maltese Falcon *by Dashiell Hammett
*Cat of Nine Tails *by Ellery Queen. (One of the first novels to deal with serial killers and still one of the best, IMO, despite having a major character who’s phonier than a three-dollar bill.)
*Point Deception *by Marcia Muller
*Trent’s Last Case *by E.C. Benson
*No Defense *by Kate Wilhelm
*The Little Sister *by Raymond Chandler (The critics are quite probably right – The Long Goodbye is Chandler’s best novel – but in this one Marlowe has to deal with a bevy of femme fatales, and the ending offers a beautiful example of the moral ambiguity that is such a feature of noir.)
*An Unsuitable Job for a Woman *by P.D. James (how I wish she would write more Cordelia Gray novels)
*Silent Joe *by T. Jefferson Parker (one of the best crime/mystery writers today, even if the MWA were tripping their balls off in awarding the Edgar to a mediocrity like California Girl)
*The Zebra-Striped Hearst *by Ross McDonald
*The Moonstone *by Wilkie Collins is one of the first, if not the first, detective novels, and, IMO, it is still one of the best.
Dick Francis is just an excellent author. I’ve read about 15 of his novels and have liked them all. I will also re-emphasize that, with the exception of California Girl, Parker is one of the best authors around today.
When it comes to series, I second Marcia Muller’s Sharon McCone novels. The vast majority of them are quite good, and McCone is one of the most likeable detectives out there.
Some of Martha Grimes’s Richard Jury/Melrose Plant novels are quite good, but beginning with The Old Contempibles, the series went south in a big way. I would not read any of these novels written after *The Old Silent *with the exception of The Lamorna Wink.
Kate Wilhelm’s Barbara Holloway novels are exceptional.
I cannot recommend highly enough Michael Slade’s series of novels about the RCMP’s Special X division. Be warned, though, these novels can also be considered horror novels, and the blood-letting gets quite graphic at times
When it comes to hard-boiled fiction, James M. Cain seems to be forgotten today, but he wrote some classics.
When it comes to Sherlock Holmes, I highly recommend *The Adventures of Sherlock Holmes *and The Return of Sherlock Holmes. At least half the stories in each book are classics, and nearly all are worth reading. *The Memoirs of Sherlock Holmes *contains “Silver Blaze,” which is of my favorite short stories and a work I regard as perfect for its genre. When it comes to the novels, *The Hound of the Baskervilles *is the only one really worth a damn.