I spy I love you, by Jean Marzollo with photographs by Walter Wick.
Full of simple riddles that are paired with ten Valentine's Day-themed photographs. The book encourages readers to find hearts, candy, notes, toys and other items crowded together on the page. Sounds easy - but it does require searching. Great for early readers and non-readers who can search for items that someone reads to them.
A Charlie Brown Valentine by Charles M. Schulz
What's a holiday without a Charlie Brown special? This story is based on the TV special in which Charlie Brown has a crush on the Little Red-Haired Girl - but she doesn't know he exists! As the Valentine's Dance gets closer, Charlie Brown discovers he has admirers of his own... but he's only got eyes for the Little Red-Haired Girl. With his luck, he'll probably end up dancing with Snoopy!
Got a kid who loves Nate the Great mysteries? Then check out Nate the Great and the Mushy Valentine by Marjorie Weinman Sharma. Nate hates mushy stuff, but then he spies a big red paper heart taped to Sludge’s doghouse. Who left Nate’s dog a secret valentine? Nate will solve the case, and then there will be no more mushy stuff. At least that’s what he thinks. . . . and then he finds himself involved in another case of missing valentines.
Valentine's Day, written and illustrated by Gail Gibbons. This is part of her "holidays around the world" series. In this book she describes Valentine's Day customs and historical origins - and includes some traditional activities such as card-making and how to make a valentine box (do kids even do that anymore?).
Arthur's valentine, by Marc Brown.
Who sent Arthur that valentine? His wrong guess about the identity of the secret admirer leads to teasing by the other children - but clues in additional messages allow him to solve the mystery.