Fun with Shapes by Joanne Wylie and Ruth Ruhman
I had a lot of clown-related stuff around when I was a kid. 90% of it still gives me nightmares to this day (you know I am talking to you weird wooden carved hobo clown wall hanging with the crying flower!)
The one clown-related thing that gave me 100% joy and happiness was the clown-centric Fun with Shapes by Joanne Wylie and Ruth Ruhman. The book teaches kids about shapes and stars some adorable clowns.
Either this book is not very popular now or was super popular back when it was printed in 1972, because you can pick up a used copy for under 8 bucks on Amazon.