"Crystal Soldier" is great.
You're totally right about a-ha moments. It totally back-fills how things came to be. Between "Crystal Soldier" and "Crystal Dragon" you really can appreciate how various threads have been drifting through the other books, all tying things back to the beginning. The book "Balance of Trade" takes on a whole different meaning after the two Crystal books.
I absolutely loved the way the progression of the Trees was depicted, and the young Tree itself.