Five Kingdoms #3: Crystal Keepers by Brandon Mull

Cole and other members of the rebellion travel to Zeropolis to free Constance, another princess imprisoned by the king. Cole's powers are still damaged, but he finds another old friend kidnapped from the real world. The Crystal Keepers are a secret group opposing the king, but all of their hiding places are being destroyed. No one knows how the locations were discovered, and the situation gets worse. Cole delivers a message to the queen, but he also witnesses some terrifying news about Owandell, a man slowly taking control of the kingdom. On his way back to Zeropolis, Cole joins up with a very unlikely ally, and they are able to find Constance. However, another enemy is revealed that endangers all of the machines and humans living in the city.

The plot continuously presented bigger and bigger obstacles that kept the suspense high. The characters had to travel stealthily to avoid capture, their safe havens became dangerous, and new, more powerful enemies were added. The author introduced two character from Cole's past, Cole had completely forgotten about one of them, and their help was critical. The author shows great imagination through his characters' abilities, and humans freely interact with machines and robots. I always enjoy Mr. Mull's books, and I'm looking forward to reading the next book in the series.