Complex sentence: However, apart from this, little is known about Oona, and her background remains a mystery (until the second book) until it is discovered that she is the daughter of Lester Lui, the leader of the Fu Tsang.
Split into simpler sentences: However, apart from this, little is known about Oona. Her background remains a mystery until the second book where it is discovered that she is the daughter of Lester Lui, the leader of the Fu Tsang.