The Valentine Mystery by Joan Lowery Nixon, 1979.

Someone leaves a mysterious, unsigned valentine for Susan Connally at her apartment on Valentine’s Day. Even though everyone was home at the time that the valentine was delivered, the only person who saw the person who brought it was Susan’s little, two-year-old brother, Barney.  All Barney can say about this person is that “He had watches on his tennis shoes.”

Susan and her other brother, Mike, decide to ask some of the other people who live in their apartment building if they know anything about the valentine or a strange person who wears watches on his shoes. 

Nobody knows a person who wears watches on their shoes.  Most people aren’t even wearing tennis shoes.  They’re wearing boots because it’s snowing outside. The questions the kids are asking about people with watches on their tennis shoes sound so strange that one of their neighbors, Mrs. Pickett, thinks that the kids are trying to find the solution to a riddle and keeps guessing things like “a spotted dinosaur who has time on his hands?”  (Mrs. Pickett is one of my favorite characters in this book. All of her solutions involve a spotted dinosaur, for some reason.)

There is a boy named Pete who lives in their apartment building.  His family has only moved there recently, and he’s in Susan’s class at school.  Susan thinks it would be nice if the valentine turned out to be from him, but he’s not wearing tennis shoes or watches.  He’s also wearing boots with round buckles on them.

The breakthrough comes when the kids discover that their little brother is going through a phase where he calls all kinds of shoes “tennis shoes.”

The book is available to borrow and read for free online through Internet Archive.

I read this book when I was a kid and then spent years trying to remember what book involved a kid who thought someone had watches on his tennis shoes before I found it again. It’s a cute, fun Valentine’s Day mystery story, and I love all of Mrs. Pickett’s guesses about who would wear watches on their tennis shoes.