The first works I ever saw of R. Gregory Chrisitie's were the illustrations for The Palm of My Heart: Poetry by African American Children. He's illustrated many books (you can find Joe in Hot City). I only recently discovered his web site and that you can buy his "gregarious art" pieces.
Christie's lines and bold colors remind me of Jacob Lawrence's work, but where Lawrence spreads his attention across the canvas, Christie—at least in the paintings that appeal to me most—homes in on faces. I love his faces: squints, smirks, side cast glances, attitude, wisdom, resolve, and peace.
[There are some really phenomenal artists that illustrate books for children. I also really like Leo and Diane Dillon (regal, clean, surreal), David Diaz (angular, signature eyes and nose), Raul Colon (soft and scratched), and Vera B. Williams (bright and joyful).]