Amos and Boris - William Steig
What on earth could a mouse and a whale have in common? Quite a lot it seems. And when the mouse is in dire need of a helping hand it's quite obvious how a large and powerful whale might help. What's not so obvious is how a tiny mice might be able to help a whale. A very sweet story of unlikely friendship and enduring loyalty.

I find that Steig's books hold a lot of appeal for children for their humor, wisdom, simplicity and complete lack of condescension. Definitely recommended!