This is book 1 of the "Young Wizards" series.

This note contains spoilers for the book.

Rita discovers she's a wizard by finding the book of this title in the library.

And meets Kit, who is also a brand-new wizard. And end up going to an alternat Earth to recover The Book of Night With Moon (which is the Word, or words, that hold the universe in existence), and in the process end up magically revising the Lone Power.

It's a pretty head-first dive into their new profession. They've barely met the Advisories for their area, and had no training beyond what they read in the manuals.

I am, however, a sucker for books that let younger people do things.