Axel the Truck: Rocky Road by J.D. Riley, illustrated by Brandon Dorman, is an infant/preschooler-aged picture book with accompanying audio, free courtesy of publisher HarperCollins.
Linkage to the freebie, which is a NOOKcolor book from B&N and will only work on that device and the iPad NOOK app, I believe.
Credit where credit is due, I found this via (and cribbed the description from, since it seems to be missing on the actual product page) the volunteer-maintained
Free NookBook Summary blog which does a nice job of keeping lists of all the available unexpired freebies which new users may have missed.
Description
Axel the Truck has big wheels, and he loves to drive fast. A trip to the mountains includes some monster hills and bumpy roads! Features read-to-me audio, perfect for sharing.