Ditto the above posts for anything by C.S. Lewis or Roald Dahl.

The "Redwall" series is popular, as are "Harry Potter" and "Series of Unfortunate Events."

Rick Riordan's books are really popular with my daughter and her friends (The Lightning Thief, Sea of Monsters)

"The Guardians of Ga'hoole" series--although a little dark

"Artemis Fowl" is good; we haven't read the others in the series

The Mixed up Files of Mrs. Basil E. Frankweiler

Celia