The Death of Koschei the Deathless.
If you did not know, the inspiration for the Three Princesses trilogy comes from an old Russian Fairytale, "The Death of Koschei the Deathless." Being that it is a fairytale, it is public domain and can be read on Project Gutenberg. It can be found as part of the fairytale collection The Red Fairy Book.
Check it out here: http://www.gutenberg.org/files/540/540-h/540-h.htm
It is the 3rd fairytale listed and can be navigated to on the table of contents.
What are your favorite fairytales and Fairytale retellings.