Dragon's Green / Scarlett Thomas.

Enhanced descriptions from Syndetics:

"This middle grade series starter is tailor-made for Harry Potter's fans." -- Kirkus Reviews

For fans of the Land of Stories and the Wings of Fire Series, this first enchanting adventure from acclaimed novelist Scarlett Thomas is set in a wondrous realm where magic most decidedly exists, a growing evil lurks, and a group of children is destined to save the world.

Effie Truelove believes in magic, as does her grandfather Griffin (although he refuses to do any magic, let alone teach Effie how to use it). After a mysterious incident leaves Griffin close to death, Effie is given an unusual silver ring and told she must look after her grandfather's library of rare and powerful books. But then the books fall into the hands of shady scholar Leonard Levar, and Effie is propelled into the most dangerous adventure of her life.

Now, Effie and her friends--nerdy Maximilian, rugby-mad Wolf, helpful Lexy, and eccentric Raven--must discover their true powers if they are to get the books back. And Effie alone will have to travel to the Otherworld, where she will uncover the true meaning of the strange old book called Dragon's Green ...