Three tales from a mystery master whose "literary distinction lies in the combination of love, humor and murder that she wove into her tales" (The New York Times).   The Episode of the Wandering Knife: What's a mother to do? When her daughter-in-law is slashed to death, the first thing is to hide the hunting knife that's sure to implicate her innocent son. But it doesn't stay hidden for long. It's just turned up in a second victim, only to vanish once again. Whatever the cunning motive is for the ghastly crimes, the game of hide-and-seek with a deadly weapon is just beginning.   The Man Who Hid His Breakfast: A woman's been found strangled in her bed. The only other person in the house is her daughter, Emma. Given Emma's motive for wanting to escape the clutches of her domineering mother, the case seems open and shut. Except Inspector Tom Brent insists Emma couldn't possibly have done it. His career depends on proving it. And it all starts with a very peculiar breakfast.   The Secret: Hilda Adams, the Homicide Bureau's undercover "Miss Pinkerton," is enlisted to investigate the odd behavior of Tony Rowland. The woman has suddenly broken off her engagement to a man she loves, crashed a car, and now keeps her elderly mother locked in her room. Does the Rowland family have reason to fear the neurotic woman? Or is Tony herself the one who's afraid? If so, of what?