All of Robert's labyrinth books have been or soon will be updated and expanded. See the following titles.
EACH TITLE IS A LINK TO AMAZON.COM
Most recently, the largest of Robert's labyrinth books,
This book describes labyrinths in church-appropriate language. It is in question and answer format.
Not yet updated and currently out of print:
Origin, Symbolism and Design of the Chartres Labyrinth
Some of the proceeds from the sale of these labyrinth books go to Veriditas, the voice of the labyrinth movement.