In studying correspondence-based picture books, I found numerous possibilities: one single letter, one-way correspondence, back-and-forth letters between two characters, letters between many different characters, and letters used in combination with a narrative (like my favorite, Click, Clack, Moo: Cows That Type).
Few picture books use this potentially-tricky format—to me, that indicates an opportunity!
Meerkat Mail by Emily Gravett
Dear Mrs. LaRue Letters from Obedience School by Mark Teague
The Gardener by Sarah Stewart, Illustrated by David Small
Dear Tabby by Carolyn Crimi, Illustrated by David Roberts
Chronicle Books is giving away a copy of Interstellar Cinderella to one lucky winner at the conclusion of ReFoReMo. Thank you, Chronicle! To be eligible, you must be a fully registered participant, comment on every post, read daily, and keep records of your progress.