Grandma has this book, Trucks and Diggers: One to Ten : Counting from Caterpillar, which the kid LOVES. Turns out it's a part of a licensed series of Caterpillar-branded books that feature only official CAT equipment.
They're obviously designed to teach your little digger the superiority of US-made heavy machinery to its foreign competition. And if the kid learns a few things about counting, letters, and colors [mostly about yellow], it's an added bonus.
Check out the Caterpillar Licensed Merchandise series, including boardbooks like Trucks and Diggers: One to Ten : Counting and the hardcover C Is for Construction: Big Trucks and Diggers from A to Z at Amazon [amazon via grandma]