My wife doesn't like books centered around friendships between children and animals. It never seems to go well in the end. Boy meets dog. Boy and dog become best friends. Heroic dog meets an untimely end. We could even create a euphemism for such literary creatures. "Jasper was a good dog until he got Stone Foxed!" (You could choose Old Yeller'ed or Red Ferned) So perhaps we need to change this relationship a bit.
I nominate Fly Guy. Tedd Arnold's I Spy Fly Guy is the seventh book in a series of early readers that feature a friendship between a boy and his fly. How could this go wrong? Actually, it does for a while. Buzz (the boy) and Fly Guy (the fly) are playing hide-and-seek. Fly Guy hides in a garbage can whose contents are immediately dumped into a garbage truck. Buzz is in panic mode and cajoles his dad to drive him to the dump. You have to like a book where the setting of the climax is a garbage dump. All is lost until Buzz realizes he has one last trick up his sleeve.
When you can find an early reader with a good sense of humor, keep it. I Spy Fly Guy's humor is very appealing and Arnold keeps the text at a level that makes it quite accessible. This would be a great book for older readers who need to work on fluency and don't want to be seen with "a baby book" in their hands.