One of my all-time favorites, "Till We Have Faces" is unique among Lewis's works. It is a dark, complex retelling of the myth of Cupid and Psyche, focusing not on the beautiful Psyche, but rather on her ugly older sister Orual.
Lewis creates a beautifully realized world, a gritty land of myth in which threads of truth are woven into the tapestry of paganism. Like his world-building, Lewis's characters have great depth, all being capable of both good and evil. Orual in particular is one of my favorite characters ever: ugly, strong, loving, selfish, courageous... A warrior and a leader, in defiance of the physical unattractiveness that caused her to be deemed "worthless" as a child. She makes big mistakes, and she does great things, and in the end her life is defined by love in ways she didn't even realize. Anyone who doubts Lewis's ability to write well-rounded female characters should meet Orual.
When pitching this book to others, as I often do, I tell them not to be intimidated by its depth and complexity: yes, it is different from Lewis's other works, and slowly paced in places, but it is a beautiful tale and well worth discovering.