Keys to the Demon Prison

By far the Keys to the Demon Prison is the strongest book in the Fablehaven series. In it, Mull corrects many of his earlier mistakes, by creating nuanced and interesting characters on all sides. He highlights shifting alliances and the difficulties arising from allying with those currently convenient. Of particular note is the comparison that Mull draws between Seth and the Sphinx in the early part of the book.

The plot is also entertaining, with even more creative settings and creatures. Some of the flaws that plagued the early books, such as villain monologuing, plot that turns out to be largely unnecessary, and repetition of exposition to each character in turn, are still present in Keys to the Demon Prison, but in attenuated form. Mull also tries to actually highlight the strengths and weaknesses of each character, rather than play on gender norms as was the case in early books.