As a second grade teacher, I became acquainted with the 39 Clues series through my classroom book club. However, my 9 year old granddaughter is really the one who inspired me to start reading the books. After reading Book One, I was hooked. The books are filled with fascinating details about history and geography. The story line is well written and fast-paced. I can't put them down! These adventures are perfect for strong readers ages 9 and up.