"Reading one book is like eating one potato chip," she said. (Mrs. Lesser to Nita.)
So You Want to Be a Wizard by Diane Duane. The library is a refuge for Nita. She has read, more than once, all books in the junior section. One day, when trying to hide from Joanne- the bully - she was surprised to find a book she'd never read. It was part of the So You Want to Be a ... This time it was So You Want to be a Wizard. At first Nita felt silly, believing such thing. But, there was no harm in trying it out. From the moment she took the Oath her life completely changed and she now follows a path of adventure and different universes. She must fight for Earth.
The book was first published in 1983. It isn't old enough to be a classic, but it is old. Surprisingly the only relation to its age can be found with the number next to the date published.