Resources:
"The Pocket Encyclopedia of Impressionists", 2003, Bookmart Ltd, England
Includes the works of Monet, Pissaro, Manet, Degas, Renoir and Sisley. With over 160 color pictures and authoritative analysis of the artists and their influences.
Sources from the Web:
Books from Amazon.com:
Degas by Himself
Degas: Beyond Impressionism
"Degas" 
"Edgar Degas: Paintings That Dance"
"Edgar Degas: Dancers and Nudes"
"Edgar Degas: The Last Landscapes"
Degas' Drawings of Dancers
Degas and the Little Dancer: A Story About Edgar Degas
Marie in Fourth Position: The Story of Degas' "The Little Dancer"
I Dreamed I Was a Ballerina