Gate of Darkness, Circle of Light is by the eloquent Canadian author Tanya Huff. The first book I ever read by her, it had me by the first chapter.
The blurb off the omnibus collection of Gate of Darkness, Circle of Light and The Fire's Stone reads:

The Wild Magic was loose in Toronto, for an Adept of Darkness had broken through the barrier into the everyday mortal world. But even in this age when only fools and innocents still believed in magic, there were still a few champions of the Light: Rebecca, who some called simple yet who was gifted with the Sight; Roland, a street musician who might one day be a Bard; Mrs. Ruth, a bag lady with some unexpected tricks in her bags; Daru, an overworked social worker; Tom a cat of rare vision and courage; and Evan, Adept of the Light, summoned to stand with them in the desperate battle to keep Earth from falling into final, total Darkness...

If you like contemporary fantasy, read this book. You will laugh. You will cry. You will swear at her editors for not letting her write the sequel... And then you will go out and buy more of her books.