Lots of Dots by Craig Frazier - Bright colors and a good introduction to the circle shape. It can be found anywhere from the buttons on your shirt to the stars in the sky. Have the kids find as many dots in the room as they can after you read the book.
Perfect Square by Michael Hall- Think "Press Here." A square gets torn up into all sorts of shapes that are turned into flowers, parks, and oceans. A very creative book that can show kids how a square can be broken down and turned into anything...with a little imagination. A great craft activity to accompany is right from the book. Give each child a square of tissue paper, have them rip it up into 6-8 pieces. Glue those pieces onto a piece of paper. Give them crayons and let them draw flower stems and leaves to make the bits of tissue paper the flowers.
Shape by Shape by Suse Macdonald- Can also be used in a shape themed storytime. By using cutouts and naming the different shapes we finally get to find out what the animal featured in this book is by the last page.