Be aware. As of the date of this review there are two Kindle versions of this book listed, and they don't appear to be connected yet. This one is from Scholastic and costs $9.99. The other is from Egmont and costs $5.98. I don't know how this happened, but just fyi. I posted the following review on both sites:
I like Garth Nix's work. The "Keys to the Kingdom" series is remarkably creative and well done, with an engaging hero and a complicated but comprehensible alternate world. The "Seventh Tower" series is action packed, and based in a coherent, complex and imaginatively created world. The problem with both series is that they were awfully padded out in the middle, and could easily have been, at most, trilogies. When I read the first Troubletwisters book I thought we were in for another good start-slow middle-strong finish opus.
Well, the very good news is that this book two is, if anything, better than book one. In book one the twins were awfully dense about their developing powers and the adults were miserly with explanations, so that you were frustrated by how slowly the story was being set up. This book two takes off like a rocket, with a lot more action, character development and advancement of the story. (Although, chapter one has a lot of narrative explanation of the back story from book one. I like that because, since I read a lot of similar books, it reminds me of where we are, what happened before, and who the major characters are. Some authors try to weave that info into the story and dialogue, but I prefer a straight intro that gets that out of the way.)
The twins remain appealing characters, and the sibling rivalry angle has been mostly abandoned. The introduction of a new character, Tara, adds some nice variety. Like everyone else I'd like a villain a little more developed than "the Evil", but actually that works better than a complicated and lame villain backstory. I'm always surprised and pleased by how advanced the vocabulary is in Nix's books, and how much regard he has for the sophistication of his fans' reading ability, and that holds true here. The books aren't "overwritten" or flowery, they just seem to read at a slightly higher level than you might expect. There is no talking down to the readers in these books.
So, while we aren't breaking any new ground here, this is a nice addition to the Troubletwisters universe, and a perfectly fine, indeed better than average, fantasy/action/adventure.