I've been researching parody for some time now, and I THINK that my book idea is legal (or at least that it would stand up in court). From what I understand, parody using copywrite/trademarked material is allowed under fair use, even though there are contradicting references: (these ones about trademark use)
http://www.ezboard.com/store/help/copyrightandparodylaw.html
http://www.phillaw.com/html/trademarks.html
Unfortunately, it seems that many companies like to sue for liable against parodies (although as long as the parody isn't deemed damaging to the intellectual property, the parody-er usually wins). The novel "The Wind Done Gone" got away with using the Gone with the Wind characters. There was a lot of hoopla, but it won out anyway.
My question, more particularly is this. Let's say that I am writing a parody of modern detective novels. I use popular characters from a variety of novels to make social comment on the genre, individual novelists, and society in general. Okay, from what I've read, I'm 99% sure that I can get away with using these trademarked characters . . . but will a publisher take a risk on publishing material that might end up with me in court? Sure, all publicity is publicity, but is it going to cause a major hindrance?
Secondly, what if it isn't BLATENT American parody? What if it is more dry satire? That's just as covered by "fair use" as parody, isn't it? It should be considered the same thing.
Out of curiosity, who are you parodying? If you parody someone like Perry Mason/Miss Marple, it would probably be different than a Law and Order/CSI character parody.
If on the otherhand your intention is more like poking fun at the whole genre, then I would follow Survivor's advice and keep this a little more vague.
Or, you could call it something like TO PROTECT THE INNOCENT (TO PROTECT THE GUILTY?), and use names that hint at or parody the characters or their authors. (Instead of Nero Wolfe, call him Caligula Fox or Stout King; instead of Sherlock Holmes, call him Hemlock Doyle; instead of Perry Mason, call him Appleby Stoneworker.)
The danger with those examples is that they may be too much like British crossword puzzle clues, but if you describe them well enough, people will "get" them anyway.
Sam Diamond = Sam Spade
Miss Marbles = Miss Marple
Dick & Dora Charleston = Nick & Nora Charles
Milo Perrier = Hercule Poirot
Sidney Wang = Charlie Chan
Generally, the best policy to follow in parodies is to have your characters sufficiently similar that it is easy to recognise whom they parody, but not so similar that they are difficult to distinguish.