Ah. Patricia Wrede. One of the only authors who can make me laugh out loud (others include Brandon Sanderson, Andrew Peterson, and Richard Thompson). She does write the best YA fiction (well, except for the Rithmatist).
I personally find that YA are (usually) children's level reading with protagonists aged 13-20, regardless of setting and/or content. I don't think it's a particularly good way of categorizing fiction.