Elisha Cooper

A 2018 Caldecott Honor book
There was a cat
who lived alone.
Until the day
a new cat came . . .
And so a story of friendship begins, following the 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 the act of moving on.