How I Became a Pirate by Melinda Long. Illustrated by David Shannon.
A story about a boy who 'joins' some pirates and he helps them bury their treasure ... in his back yard...complete with a treasure map with an X on it! Fun book.
X Marks the Spot by Lucille Recht Penner.
The story tells about a time when brothers Leo and Jake move into their grandfather's old house, and they find several treasure maps. Each map is a chart that they must follow, block by block, to discover the many treasures of their new hometown. It's part of her Math Matters series.
Alphabet Mystery by Audrey Wood.
It features the letter "X" and is a really cute story!
Little x is missing from Charley's Alphabet, and the other lowercase letters go off to solve the mystery of his disappearance, learning in the end how valuable a little x can be.
Toot and Puddle: A Present for Toot by Hollie Hobbie.
The bird in the story is exotic, expensive, extraordinary, etc.