So You Want to Be a Wizard (by Diane Duane)

Nita, chased by bullies, hides in a library and finds astrange book called So You Want to Be a Wizard. It contains information about the nature and structure of the universe, instruction in the use of magic, and a Wizard's Oath. After taking the Oath, Nita discovers that thecontents of the books change to meet her needs and growing abilities, and that she must undergo an ordeal.

She meets another young wizard-to-be, Kit, and together with an intelligent white hole they accidentally call up, they are transported to a dangerous parallel universe run by the Lone Power, where they must retrieve the Bookof Night with Moon which he has stolen and which is necessary for maintaining the stability of the universe.

Long before Harry Potter went to Hogwarts, Nita and Kit began their wizard training in this unusual series that blends fantasy and science fiction in equal measures. Rooted both in Judeo-Christian cosmology and a sort of all-encompassing animism, the novel's fascination comes not only from the weird adventures of the protagonists, but also from the magical-scientific details, conveyed in the author's own delightful technobabble.

This entry in the series is also a love song to New York City, and the world of the Lone Power is seen as a twisted version of Manhattan filled with living (and mostly vicious) taxicabs and helicopters. The settings, in and around downtown Manhattan, are vividly realized.