Dirty Tricks — Emily Rodda

Another unpredictable adventure from the internationally bestselling author of Deltora Quest!

The Help-for-Hire gang's toughest challenge yet begins when a new teacher assigns too much homework. The local library seems like the best place for them to hit the books . . . until the books start hitting back! It turns out the Phantom of the Library is playing practical jokes on innocent readers, and Richelle's not laughing.
As the tricks become more and more treacherous, Richelle and her five loyal friends must unmask the prankster soon, or else the library will be destroyed forever. . . .