I really enjoyed Shade's Children. It is fast-paced, full of action and intrigue. I cared about the 4 youngsters as they struggled to survive, and I was deeply immersed in the strange future world that the author created. My big disappointment was that no logical explanations were offered as to how and why such a world came into existence in the first place. Who are the Overlords? Why are the kids endowed with special abilities? There are just too many questions left unanswered. I have read many futuristic novels like this, The Giver being one of them, that left me very unsatisfied, and I think probably a plausible explanation was beyond the authors.
The Seventh Tower series contains 6 books and is re-published in 2 hardcover volumes in the US. I just finished the first volume, containing the first 3 books, and I enjoyed it enough that I want to finish the series. The Keys to the Kingdom series, on the other hand, is just way too far-fetched and fantastical that I did not even want to suspend my belief to enjoy it.