The Monster at the End of the Book by Jon Stone
Leonardo the Terrible Monster by Mo Willems
Go Away, Big Green Monster by Ed Emberley
Where the Wild Things Are by Maurice Sendak
My Monster Mama Loves Me So by Laura Leuck
I Just Ate My Friend by Heidi McKinnon -- It's surprising but it's growing on me, although maybe some would think it a little dark . . .
There is a Monster in Your Book by Tom Fletch is newer and a fun read.
The Color Monster: A Story about Emotions by Ann Llenas has a good yoga connection.