The Pie Wagon by Lillian Budd, illustrated by Marilyn MIller

This is one of my ALL TIME favorite books from my childhood. Unfortunately, it is out of print. If you are lucky, you might find one of these in your library. If you do, read it for the joy of an old book with illustrations from what must be the turn of the century (as in, 1900).

A little girl named Tilly hears the skinny Pie Man calling out, "Pies! Any pies today?" Townsfolk gather to see what the pie man has brought. The little girl has the privilege of stepping into the narrow wagon to spy the rows of pies, labeled alphabetically, A-Z. She reads each pie name, but is stumped when she sees that there is a pie next to X without any name.
"What's the name of the pie in the X rack?" she asked.
The skinny Pie Man laughed. "Can you think of the name of a pie that begins with X?"
The little girl thought and thought, but she couldn't think of a pie that began with X. 
"I couldn't either," said the Pie Man. "So I use this rack for the eXtras--eXtra big and eXtra delicious. Today it's banana cream with strawberries on top."
"Oh," said the little girl. "That's the one I want!"
And she took it home to her mother.

I haven't met a child yet who hasn't liked to read this book because they want to see all of the kinds of pie in the wagon. It's a perfect ABC book, too, for that reason, since you want to read it over and over (as you dream of the kinds of pie you'd like to taste!).