The Rat and the Tiger

``I'm a rat, just a tiny little rat,'' says the diminutive narrator of this piquant tale about friendship. Towering above him is his best buddy, Tiger, ``a big tough fellow.'' Rat explains that the two ``used to have a little problem.'' Whenever they played cowboys, Rat always had to be the bad guy. And whenever they shared a doughnut, he was left with a small piece while his buddy received the Tiger's share. But one day the big bully goes too far. When he kicks over the elaborate block castle Rat painstakingly built, the latter declares that Tiger is no longer his friend. And the stubborn little fellow makes sure that his companion has truly mended his selfish ways before he declares them pals once again. As in her past books, Kasza ( The Pigs' Picnic ; A Mother for Choco ) here delivers a valuable lesson cunningly and effectively. With their drolly expressive faces, Tiger and Rat pass on the importance of sharing, playing fairly and treating others as you'd like to be treated. As a bonus, Kasza adds a characteristically clever tangle at story's end. Ages 4-8. (Mar.)