Croggon, Alison. The Riddle. Cambridge: Candlewick Press, 2004.
This sequel to The Naming is also absolutely excellent. I don't generally enjoy super-thick novels with multiple sequels, mainly because the stories often sound so very similar to each other that it's like reading the same book with a slightly different setting. This series, however, is much more Tolkein-esque and plotted and planned to be true sequel. I enjoyed this book all the way to the end and am excited to start the next book and continue following Maerad's journey to find the split song. If you haven't read The Naming yet, start with that one, but then read this book as well.