These are all very good picture books for children that feature monsters. I will add to this list as I find more.
Love Monster by Rachel Bright
Monsters Eat Whiny Children by Bruce Kaplan
The Monster’s Monster by Patrick McDonnell
Even Monsters Need Haircuts by Matthew McElligott
Some Monsters are Different by David Milgrim
Frankenstein Makes a Sandwich by Adam Rex
Wanda’s Monster by Eileen Spinelli