Much better than the Keys to the Kingdom series, this series involves two quite deeply and intricately-drawn fictional worlds, complete with their own complex cultures. The main protagonist, Tal, may not be entirely likeable in himself at first, but he develops, throughout the course of the books and the adventures he experiences, from a unthinkingly compliant product of his culture into a person with some perspective on it and even the ability to change it.
The other protagonist, Milla, is another thing I haven't seen done well really in any fiction: a believeable, strong warrior girl/woman who the author is not afraid to poke fun at.
An extremely enjoyable series, the importance of whose core themes I think is pretty widely under-appreciated. ( )