These are all great suggestions, but I'm really surprised no one has mentioned 'Ballet Basics' or 'Ballet: Beyond the Basics' both by Sandra Noll Hammond. Although they are typically used more as textbooks, they do a great job of illustrating the movements and explaining the "whys" and "hows." For my younger students, I really like:
1. 'First Lessons in Ballet' by Lise Friedman - It's short and doesn't cover a ton of movements (only about 25), but the photographs are very clear.
2. 'The Illustrated Book of Ballet Stories' by Barbara Newman - It tells the stories of Coppelia, Swan Lake, Giselle, Sleeping Beauty, and the Nutcracker and has beautiful illustrations and photographs.
3. 'The Young Dancer' by Darcey Bussell - great photos, informative and to the point - good for a beginner.

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