The War of Undoing is, at first appearances, a pretty straightforward book. The humans and the vuma live in an uneasy (and clearly temporary) peace. [ominous thunder]
With that established, cut to...
Three children - the Rainings - living alone, unchaperoned, and in poverty in the unwelcoming city of Tarot. They receive a mysterious message saying that they're needed for a Great and Magical Cause. This gift horse seems like a truly spectacular chance. They can leave the city, pursue their capital-D-Destiny, and maybe even find - and bollock - their absentee parents.
Of course, things are never really so simple - not even in even high fantasy. The Rainings are quickly separated, and head down their own paths, making new friends (and enemies) along the way. More worrying, what they assumed was their Destiny is perhaps someone else's. The three children learn that being the instrument of a Great Cause is less about being a hero and more about being, well, a tool.
This is a long - and often quite meandering - book. There's a slow start, followed by a lot of quiet, discursive tangents. Several of Undoing's plots and 'hints' don't coalesce until the very end, and certain momentuous occasions and world-changing events - which would be the very heart and soul of other fantasy novels - are downplayed, and shifted to the background. As a result, The War of Undoing can feel frustrating at times. But, and I can't stress this enough, stick with it: this book simply has different priorities.