TRIANGLE  by Mac Barnett and Jon Klassen.
With a cover that is a bit reminiscent of The Three Robbers, this duo enlivens two potentially dull shapes with a simple trickster tale.  Triangle thinks he has bested Square until the latter gets the last laugh.  Could this be a concept book?  You Betcha, and young readers will definitely be able to recognize triangles and squares when the book is finished, but that's secondary to the surprise of finding these shapes with big personalities and big ideas.  A reminder that there is no one definition of a picture book, anything can be a character if your imagination is big enough and the pictures can be gritty , but a good story is a good story.   Original and memorable.