Every Monday, I'll be doing spoiler-free, bite-sized book/magazine reviews.
Personally, The Indigo King is reminiscent of the third season of the TV series Heroes: it's quick, it's action-packed, and manages to recapture the charm of the original novel that spawned it.
The writing of James A. Owen is easy to get into and is quite readable. Description is eschewed in favor of a more direct approach, especially when it comes to introducing the characters (and this is pretty much my complaint about the entire series so far). The story kicks off from the start and takes a darker tone compared to the previous two books that preceded it, all the while maintaining its young adult tone and style: characters, for example, tend to die off-screen if at all, and there are several blundering moments for readers to take the story too seriously.
The art of Owen is also impressive as each chapter has an accompanying illustration. The black-and-white medium suits the art particularly well.
In many ways, the third book is an improvement over the two that preceded it. The Indigo King succeeds in bringing our protagonist at the forefront of the action, instead of relying too much on the supporting cast. There's also a strong emphasis on internal character conflict that was present in the first novel and only touched upon in the second, giving the book an additional layer of depth. Then there's all the allusions to various myths, fiction, and real world events that Owen includes which gives well-read readers something to look forward to.
The Indigo King works well for its target audience and goes beyond being a competent sequel. More action, more complexity, and the stakes are larger makes this a worthwhile read, arguably surpassing the books that preceded it. If you enjoyed the writing style of Owen in his previous books, you'll do fine with this one.