The Singing by Alison Croggon
Maerad attempts to solve the last riddle of the treesong so that the Nameless One can be thwarted and peace can be restored to the Seven Kingdoms, but first she must find her brother Hem, who holds the other half to the riddle, after waging war against the Landrost.
The Singing is Book 4 of Pellinor.
Book 1 – The Naming
Book 2 – The Riddle
Book 3 – The Crow
Book 5 – The Bone Queen (not available at this time.)