The Leaf Thief by Alice Hemming.
This is a cute story and my kids and I chuckled quite a bit as we read it out loud together. Also, the illustrations are just lovely and perfect for fall reading.
From Amazon:
Perfect for fans of Fletcher and the Falling Leaves, The Leaf Thief is a funny picture book that teaches kids about autumn, adapting to change, and the seasons.
Squirrel loves counting the leaves on his tree--red leaves, gold leaves, orange, and more. But hold on! One of his leaves is missing! On a quest to find the missing leaf, Squirrel teams up with his good friend Bird to discover who the leaf thief could be among their forest friends.