I only have a couple picture books that include a couple cute illustrations etc. I'm sure "DK Eye Witness" would have published something on turtles or tortoises. That would be worth looking into. Those are usually pretty good books.
The 'Owen and Mzee' books are aimed for about that reading level (although probably on the lower end). 'The name of the tree: A Bantu tale' is purely fiction, but also lovely if you're looking for tortoise protagonists.