This is one of my favorite retellings, ever, as well. Lewis offers a stunningly complex characterization; the story of Psyche and Eros, told from the point of view of one of the "ugly sisters." And, honestly, I was blown away by this. This book is one of the best books I've ever read--being both in my favorite category of story and amazingly, beautifully written.
This book makes me long for more mythology and fairy tale retellings from Lewis... I guess I will just have to read the rest of his books, instead!
I cannot recommend this enough. Whether or not you like the Narnia books, if you love mythology and retellings, you need to read this book.