I'm Sorry You Got Mad by Kyle Lukoff, illustrated by Julie Kwon
A book about the difference between a real apology and the non-apology we've all been on the receiving end of. ("I'm sorry you got mad" being the all-time classic.) Useful for kids, frankly useful for adults, and a good one to revisit a few times a year.
Turtle in a Tree by Neesha Hudson
Two kids disagree about whether the thing they're looking at is a turtle or a dog. I often help end disagreements in our house with, "Is it more important to be loving/kind/happy/generous, or to be right?" This book brings that lesson to life better than any lecture I've ever given.
Kindness Club: Squirrel Learns to Forgive by Ella Law, illustrated by Laura Vitória Jager
I almost didn't include this one because it felt a little too on-the-nose to me. But as didactic as I found it, my boys both absolutely loved it, gave it the rare double thumbs up, and made me promise to put it on the list. They win.
Big Red Lollipop by Rukhsana Khan, illustrated by Sophie Blackall
A girl gets invited to her first birthday party, and her mother insists she bring her little sister along. The little sister ruins everything. Years later, when the little sister gets her own first invitation, the older sister has a choice to make. The forgiveness here is so real you almost miss it, which is exactly why it’s perfect.