The Giblin Guide to Writing Children's Books by James Cross Giblin (Writer's Institute Publications, 2005)(fourth edition--revised and updated). Giblin's Guide highlights the various forms, including non-fiction, fiction, ages categories within fiction, types of fiction, picture books globally, and rhyme in picture books specifically. It also features information on "from submission to contract" and "from contract to publication."
Recommended as a companion to What's Your Story? A Young Person's Guide to Writing Fiction by Marion Dane Bauer (Clarion, 1992) and especially to children's non-fiction writers.