Prada and Prejudice by Mandy Hubbard.*
In this Austen spin-off (very loosely based on Pride and Prejudice) for young adults, high school sophomore Callie goes on the class summer trip to London, partially to try to change her D-list image, and partially to avoid visiting her father and his new family.
In an effort to impress the Popular Girls, Callie purchases a pair of red Prada shoes. Unused to wearing such high heels, she trips as she leaves the store. She hits her head and wakes up in 1815.
Not sure where she is or what to do, she is scooped up by Emily, who is expecting a visit from her American friend Rebecca (whom she hasn't seen in years) and mistakes Callie for her. Callie becomes fond of Emily, her 19-year-old cousin Alex, the Duke of Harksbury and his mother, Victoria. When she discovers that Emily is betrothed to a man 20 years her senior, she is horrified and vows to break the engagement so that Emily can marry the wealthy merchant she loves.
LOTS of holes in this plot, but they don't detract from the pleasantly amusing tale, which will be enjoyed by lovers of Meg Cabot and might encourage them to try Jane Austen.
*Purchased from Sony for eReader.