From the description:
"When her mother dies, fifteen-year-old Keelie Heartwood is forced to leave her beloved California to live with her nomadic father at a renaissance festival in Colorado. After arriving, Keelie finds men in tights and women in trailer trash-tight bodices roaming half-drunk, calling each other lady and lord even after closing time! Playacting the Dark Ages is an L.A. girl’s worst nightmare.
Keelie has a plan to ditch this medieval geekland ASAP, but while she plots, strange things start happening—eerie, yet familiar. When Keelie starts seeing fairies and communicating with trees, she uncovers a secret that links her to a community of elves. As Keelie tries to come to grips with her elfin roots, disaster strikes, and Keelie’s identity isn’t the only thing that’s threatened."
4.5 stars on 44 reviews, with only 2 negatives. 336 pages, probably best for 12 and up