well, not specifically screenplays. just interrested in learning the in's and out's of detective stories. the best way to learn would read as many of those stories i can. or watch some good movies or tv shows.
*ahem* Errrrr.... Yes, I withdraw the question... must have been most inattentive, as your original post made that clear. Sorry.
Okay... All of the above are very good suggestions. There are plenty of others. A rather nifty little affectionate spoof of the genre and some of its most famous figures is Agatha Christie's
Partners in Crime, in which nearly every chapter or two sees the Beresfords taking on the role of one of the more famous fictional detectives (at least, those famous up to the time of the novel's writing in the 1920s). So you have the Baroness Orczy's Old Man in the Corner (predecessor to Stout's Nero Wolfe and others of that ilk), Sherlock Holmes, Dr. Gideon Fell (created by John Dickson Carr), etc. It can give you a feel for each one's approach without having to read a lot of novels, allowing you to look for ones you'd specifically be interested in.
A link on Fell:
Dr. Gideon Fell
And look for such anthologies as
A Treasury of Great Mysteries (2 vols.) ed. by Howard Haycraft and John Beecroft, or
A Treasury of Modern Mysteries (2 vols.) ed. Marie R. Reno. (I would have included a link with the t.o.c. of the first of these, but -- surprisingly -- didn't find one. (There are plenty of anthologies of detective novels and short stories out there, including the Mammoth Book of, etc. Any of these would be helpful in finding the types you'd be interested in. There are also a couple of books out there that are just brief descriptions of each fictional detective, with a listing of all the books in the series... I forget the titles at the moment, but you could probably find them via a local library.)
WorldCat: A Treasury of modern mysteries.
There's also the wonderful Travis McGee books by John D. MacDonald:
Travis McGee - Wikipedia, the free encyclopedia
Not your typical mystery/detective story, but -- despite some minor flaws -- excellent books, nonetheless.
And, for just a general introductory list, check out the sections on "Famous fictional detectives" and "Detective debuts and swansongs" here:
Detective fiction - Wikipedia, the free encyclopedia