On Sudden Hill by Linda Sarah & Benji Davies
I have to admit to feeling predisposed to love this book because it reminded me of the hills* near Chipping Campden in the Cotswolds where I used to teach.
Birt and Etho play everyday on the hill, best friends forever, building skyscrapers and pirate ships and watching the clouds roll across the sky. Then another boy arrives on the hill who upsets their rhythm, and Etho finds he is out of step. But friendship wins out and there is a Monster Creature Box Thing, a new rhythm and an Etho-Shu-Birt-iness that makes this one of my favourite stories about belonging and the vicissitudes of childhood.
Keep on the shelf with the Complete Calvin & Hobbes, Frog and Toad, Our Twitchy and too many others to mention. You'll need some more bookshelves.
*It looks like Mickleton Hill specifically - I used to drive past it in the morning, and on Thursdays the Rambling Club would stride out where Hidcote Gardens drops over the Cotswold Ledge.