Book of Signs (Diadem

"Our heroes arrive on a fresh world and find a new menace - a shapeshifting sorcerer - and new friends, centaurs. The trio are discovering more about themselves and their mysterious destiny."

Originally published as part of Scholastic books, Diadem is a fantastic series about three friends traveling across dimensions in search of the truth behind the universe, and their own place in it all. The author encourages the reader to try to solve puzzles along with the characters as they traverse from one planet of the Diadem to another and includes many interesting twists on 'normal' fantasy and science fiction themes and creatures.

The first six books in the series were published consecutively in 1997, then re-published by Llewellyn in 2004. the next four were published by Llewellyn starting in 2004, and the remaining two were published by Dragonhome Books in 2012:

Book of Names Book of Signs Book of Magic Book of Thunder Book of Earth Book of Nightmares Book of War Book of Oceans Book of Reality Book of Doom Book of Time Book of Games