This review will contain spoilers!
In many sentences:
There's a lot to consider for this critique. First, there is the fact that I have not recently felt much motivation to read, so some of the complaints I have about this book may originate from my current biases. However, with that said, this book is not nearly as good as the first in the series for many reasons, which I will discuss in no particular order.
One, while the first book has a very clear sense of purpose (Takeo must help Shigeru assassinate Iida), this book lacks focus. Takeo has no goal beyond surviving the Tribe's training. Even when he finally escapes from the Tribe, his goal of this book is to train and prepare for war. The battles don't come in this book, however, so it feels very pointless. There's no payoff (yet) to any of the things he does. Even the most interesting event (the marriage between Takeo and Kaede) happens in a single sentence. There is no fanfare, just a sentence saying they were married.
Kaede's own story suffers the same problem, but it's less pronounced if for no other reason than her own growth as a character (from a girl who bought into the sexism of the age, to a woman who struck fear into even the most powerful men). Still, her immediate purpose is quite similar to Takeo's in that she must learn and train for ruling as a man would. Sure, there might be a great payoff in the next book, but we're not talking about the next book we're talking about this one.
Grass for His Pillow has a lot to learn from Harry Potter in this regard. In addition to the constant character growth and overarching plot development, each HP book also had its own storyline that started and resolved itself a single book. Some were better (Goblet of Fire) than others (Order of the Phoenix), but at least they were there. Grass for his Pillow, however, feels like nothing but filler for what is hopefully a much better entry to come.
The next complaint is probably because I haven't been in a reading mood, but this book did not feel like it captured the same magic, or came from the same inspiration as the first. I never felt as captivated or enthralled by this volume. Maybe it's just because I knew all the tricks that were coming (in terms of Takeo's special powers), but it may have also been because there just wasn't that same combination of events to make it an exciting story. This isn't to say Hearn wasn't inspired, or doesn't have some great plan, but it all felt very stale and flat.
I still think this series has a lot of potential, and I will eventually get around to reading the third book, but it probably won't be any time soon. Hopefully it picks up where the first left off.