I have a ton of references and would like to share some of the best ones, here:
Joseph Sheppard's "Bringing Textures to Life"
James Gurney's "Imaginative Realism"
Picture book illustrators: Keith Baker, Yuyi Morales, Jim LaMarche, and Carol Heyer (especially her "Humphrey" books).
For less recent illustration inspiration, I look to Elsa Beskow, John Bauer, Tove Jansson, N.C. Wyeth, the d'Aulaires, Wanda Gag, and Ivan Bilibin.
Fine artists: Duane Keiser, Karen Jurick, Jeffrey T. Larson, Carl Larsson, Mary Cassatt, Alphonse Mucha, John W. Waterhouse, Degas, Gauguin, Lautrec, Klimt, Picasso's minotaurs and doves, and a Finnish artist named Askel Gallen-Kallela.
YouTube tutorials for painting: http://youtube.com (search for the media you use, there are too many to list)
Tutorials for Photoshop: http://psd.tutsplus.com/category/tutorials/illustration/
Tutorials for Illustrator: http://vector.tutsplus.com/category/tutorials/illustration/
Other online inspiration: http://muddycolors.blogspot.com/
And, last but certainly not least, the Library of Congress' Prints and Photographs Collection for historical reference: http://www.loc.gov/pictures/
Hope these fire some inspiration!