Rules for Writing Mysteries
Really, I don't know what the rules are, but here's one that I would put right up at the top if I were the person in charge of establishing rules:
Don't make your main character stupider than your readers
Unless you've established stupidity as a character trait then, maybe, you can get away with it. And even then your main character should 'get it' once they've been hit over the head with the obvious ten or fifteeen times.
Let's say, for example, that a murderer is kidnapping nannies in your neighborhood. One, you should actually get help for or listen to your crazy neighbor who is actually telling you obviously useful information before he gets himself shot in the head by police and his brains splattered all over you. And, also, at that point, you will sound totally stupid insisting that he's not the murderer (even tho the readers already totally know that he isn't) when he just threatened you with a gun, and you have already shown that you have absolutely no critical judgement ever. And, most important, the reason you don't believe he's the murderer (even though they just found body parts in his apartment)--because you just don't think he's the kind of guy who would do that--is not credible when he JUST THREATENED YOU WITH A GUN. You can have a reason but--I just don't think he would--is not a good reason at that point.
Also, if there are mass kidnapping/murders going on, you don't make your own nanny accept a ride home from a guy who's always just 'there' and who your nanny has already actually told you makes her uncomfortable. Oh yeah, and when the creepy guy next door is lurking in the basement of the psychiatric hospital (where you work so one would think you might have a bit of a clue about these things) and he says he's 'just waiting' for the radio personality psychiatrist because she's an 'old friend'. But he doesn't want her to know he was there or leave a note or go through the receptionist, he is totally stalking her!
There were things I liked about the setting and the writing and the non-stupid parts of the characters (the book, BTW, is The Nanny Murders by Merry someone who I'm too lazy to look up), but it just got to be too much--the main character's understanding basically driven by what the ending of the story demanded rather than what was actually written on the page.