This review will contain spoilers!
In many sentences:
I am still really enjoying this series. The things I've said in the past (fun, imaginative, exciting) all still ring true in this, the fourth volume. It took some time to get going this time (perhaps because the book I just read was so much more serious), but by 70 pages in I was as caught up in the story as much as always.
One thing I really enjoyed about this entry in the series is the maturation of Seth, and the realistic manner he acts, and those around him react. Seth may do some crazy things (i.e. sneaking off to talk to a Demon and break into a Centaur stronghold), but when confronted with why he did these things, his response is logical: things are desperate and he did what was necessary and what only he could to help save the world. I enjoy this because yes, Seth is prone to acting impetuously in the face of adventure, but at the same time this is the possible end of the world. Why shouldn't a boy with special skills do what he can to help out? I'm glad that the adults realized this as well - it would have been frustrating if they continued to treat him the way they did in the first book.
I also liked the play between the dark and light in Seth and Kendra. This idea could definitely represent how differently siblings often see themselves, but it was also nice to see how when they come together (and literally join hands) they become nigh unstoppable. Together they are even capable of slaying the unslayable poison dragon. I hope that this theme of siblings working together, overcoming their inherent conflicts as siblings, to accomplish great things, carries on in the final book.
In terms of the plot twists, it was interesting to learn that the Sphinx is, in fact, human. I was beginning to think he might be a former dragon, now wizard along the lines of Agad. Knowing he is human definitely makes him appear less invincible. It will be interesting to see if there are any final twists when it comes to the Sphinx in the last book. I also like that Vanessa is back, and is beginning to gain the trust of the group. I hope that she turns out to really be on their side, as she is a great character.
The final twist, that their parents have been kidnapped, doesn't seem real yet. I can only trust that Mull will pull this plot line off as well as he has all the others.
Each book so far has made me want to recommend this series more and more. The excitement, adventure, creativity, and fun of the reading each book is still going strong and I cannot wait for the final installment. I am confident I will enjoy it, not just because I have enjoyed the four others so far, but it really feels like Mull has known where he is going with the series. None of the major plot elements (Kendra and Seth's powers, the artifacts, the Sphinx, Vanessa, and Gavin's betrayals) feel forced or inconsistent with the series as a whole. I appreciate this and think it shows that the series will end on a high note. How it will end, I can't even imagine, but I'm looking forward to it.