The word of the day, thanks to Merriam-Webster:

whodunit   hoo-DUN-it  noun
        : a detective story or mystery story

In 1930, Donald Gordon, a book reviewer for News of Books, needed to come up with something to say about a rather unremarkable mystery novel called Half-Mast Murder. “A satisfactory whodunit,” he wrote. The coinage played fast and loose with spelling and grammar, but “whodunit” caught on anyway. Other writers tried respelling it “who-done-it,” and one even insisted on using “whodidit,” but those sanitized versions lacked the punch of the original and have fallen by the wayside. “Whodunit” became so popular that by 1939 at least one language pundit had declared it “already heavily overworked” and predicted it would “soon be dumped into the taboo bin.” History has proven that prophecy false, and “whodunit” is still going strong.

It’s a bit odd, really; normally phrases that are so grammatically incorrect will grate on me – the use of ‘between you and I’ in the song ‘Hungry Eyes’ in Dirty Dancing, for example! But I happily accept whodunit, perhaps because it’s been in common usage for so long.
And these days, mystery novels have expanded to include whydunit and howdunit just as much; it’s not uncommon to find out the criminal’s identity at the beginning with the story then being of why and how.