I'm reading a couple of rhyming picture book manuscripts for a beginning writer and pulling resources that may be of use to her. They include:
Rhyming Picture Books: For Those Who Must by Margot Finke from the "Musings" Archive in April 2004. This page offers a number of really on-target links. I especially like:
"To Rhyme or Not to Rhyme" by Dori Chaconas from Smart Writers Journal in April 2003.
Writing Picture Books by by Marisa Montes. She also offers more great information on Getting Published.