I've promised to make a list of some of my favorite picture books for my class to find inspiration in, so here's a very short list:
William Steig is one-of-a-kind for funny stories with invented language and a bit of danger. Of course Shrek is great, so are Wizzil, Rotten Island, Sylvester and the Magic Pebble, and Zeke Pippin.
Harold and the Purple Crayon is pure genius.
Tomi Ungerer's Mellops series are funny, offbeat, and charming.
Tove Jansson's Moomin series is beautiful.
Shaun Tan's Rules of Summer would count as a "list" book- a series of images united by a single theme, but lacking a narrative. His paintings are gorgeous and the list is imaginative.
Carson Ellis' Du Iz Tak is a gorgeous look at the cycle of life.
I recommend that my students try to read the books on the NYTimes Bestseller list for Children's Books.
Don't forget about this excellent blog, full of interviews with authors and illustrators:
Anyone can become a member of the Society of Children's Book Writers and Illustrators by paying the annual fee. The conferences, where you will hear multiple speakers and potentially have a chance to have your portfolio reviewed for an additional fee, are very pricey, but membership allows you to find others in your region and attend a variety of illustration related discussions or events and perhaps to form a writing/illustrating group to workshop your project.
Writing with Pictures, by Uri Shulevitz
Seven Basic Plots, by Christopher Booker
Children's Writers & Illustrators Market
Perspective: A Guide for Artists, Architects, and Designers, by Gwen White
Drawing Animals, by Norman Adams and Joe Singer
An Atlas of Animal Anatomy for Artists, by W. Ellenberger, H. Dittrich, and H. Baum