Unknown modern fantasy book title

It’s been so long since I read the book unfortunately so much of it has faded from memory I’m not sure I’ll be able to offer a specific enough description to be of any help but I’m going to try.

It came out in the middle 00’s, 04 or 05, I read it in 05 or 06. It was the first in what would hopefully become a series and the first book by the author as well who was also a woman. The story follows a young woman in New York City, timid and shy, a little mousy and I think she may have just started a relationship, who is approached by a mysterious older man and told that she is the inheritor of a great legacy and she possesses powers beyond her comprehension. At first she refuses to believe it as she has a decent job, assistant or low-level secretary for like a magazine or fashion company, and wants nothing to do with it. Much of the middle of the book is a blur and I hesitate to mention what I can recall for fear that I’m misremembering and the type of book it was is not usually my forte. She goes through the typical “warriors journey”, is trained by a female mentor who is a little mean (and I think she eventually dies), learns to control her powers which are sort of psionic, sort of magic and fiery. Origins are never really explained and though it seems like magic it’s never explicitly referred to as such. Eventually it culminates in a showdown with the bad guy, who’s been both trying to kill her and set in motion some kind of apocalypse, where he uses his power to infiltrate the security at an outdoor festival, very similar to Burning Man, in the middle of the desert out west. She manages to beat the guy and realizes that her life will never be the same and that she must abandon her old life. The book ends with her appearing, ghost-like, with the old guy who recruited her at the beginning, to her best friend in NYC, before she disappears again. THE END

That sounds really horrible and vague and I’m not sure if it’s enough, but if anyone could PLEASE help me find out the title and tell me if there are more books that followed, I would be very appreciative. It’s been driving me crazy for years, it only just occurred to me to ask the hive mind the other day. Thanks!

Just bumping this since it’s getting a little old. I have zero idea. But I know we have some urban fantasy fans among us who might.

Thanks, I hope it works.