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.