Big Cat, Little Cat by Elisha Cooper

There was a cat 
who lived alone.
Until the day
a new cat came . . . 

And so a story of friendship begins, following two cats through their days, months, and years until one day, the older cat has to go. And he doesn’t come back.

This is a poignant story, told in measured text and bold black-and-white illustrations about life and the act of moving on.