This book contains 5 young adult books. It is by Diane Duane. It has: "So You Want to Be a Wizard", "Deep Wizardry", "High Wizardry", "A Wizard Abroad", and "The Wizard's Dilemma".
Long before there was Harry Potter, there was Nita Callahan and her friend Kit. Nita doesn't have a highly significant scar on her forehead, nor does she live with abusive adoptive relatives. She's a perfectly ordinary girl living with her perfectly amiable family (well, her younger sister, Dairine, is impossible, but that's the way younger sisters are) on Long Island, where she goes to public school and has problems with the class bully.
Then one day she runs across one of those little books that help young people decide on a career: So You Want To Be a Doctor; So You Want to Be a Plumber; So You Want To Be a Wizard...
Nita can't resist. She takes it home; she studies it; she recites the Wizard's Oath she finds inside--and she finds herself launched on a whole series of dangerously magical adventures. They take her to the depths of the ocean, to the moon and beyond, and to other dimensions entirely. She meets and befriends (and sometimes does battle with) wizards of all sorts and species. And every so often she, Kit and Dairine save the universe.