A while back, Bibliophile (Another 52 Books) posted about Van Dine’s rules, created in 1929, for writing detective stories. My earlier comments on her posting can be found here. Bibliophile said at the time that she would return to the topic, and, hooray, she has. She has listed the rules and for each one has given her own interpretation for present-day authors.
Implicit in some of these rules, but not directly stated, is that it is breaking them to have someone killed by an unknown, untraceable poison. Christie did it at least once, and this kind of thing still happens occasionally. What a cop-out.
Unlike Van Dine, I think romance ("relationship issues" we might say these days) is very acceptable in a crime story; plenty of books have been augmented in this way with no detriment to the main business of detecting, and often adding an extra dimension — Karin Slaughter is one good example of how to do it well, though Van Dine might have opined that she uses relationship tension to spin out her plots and I could not in all honesty disagree. I think there is some truth to the observation that in detective fiction, an author can get a bit carried away by the romance aspects to the detriment of the detection (eg Dorothy Sayers and Lord Peter Wimsey, or for a more modern example, Elizabeth George, in whose novels the detection is becoming almost perfunctory).
I’m with Bibliophile in thinking that the rules "there must only be one detective" and "there must be only one criminal/crime" are alright in the main but are also OK to break judiciously. Quite a few modern crime thrillers work well by having two separate threads of plot which are not apparent as separate to the reader until quite late on. It is just as satisfying for apparently distinct plots to converge as it is for misdirection to let the reader deduce that a plot is one whereas actually it is more, so long as the author does not "cheat" the reader in some of the other ways outlined in the rules.
Apart from these exceptions, I agree with Bibliophile and the original rules: that good detective, crime, mystery or thriller fiction (whatever you want to call it), does well to operate within this framework. Oh yes, and keep it short (ideally less than 250 pages). Reader satisfaction will be the result.
At the end of Bibliophile’s post is her expansion of the original list of cliches to be avoided (the dog that did not bark in the night, type of thing). This is fun, and I hope people will add to her list of these. I am sure plenty more cliches have accrued since 1929.