The Book of Kings (#3 in the Mister Max Series) by Cynthia Voigt is the culmination of the series, though it definitely leaves itself open for further adventures of Mister Max.
In this book, Max is finally able to get to his parents. Max, who usually seems to work alone, definitely needs all the help he has assembled. With his parents in such a remote and controlled environment, Max’s options are limited.
Again, my only criticism is the age of Max and Pia. Otherwise, I highly recommend this book for any confident reader and probably not so much for the reluctant readers who might benefit from a simpler vocabulary.