The Heroines by Eileen Favorite.
Thirteen-year-old Penny Entwistle and her mother Ann Marie run a bed and breakfast inn in Prairie Bluff, Illinois.
Penny is forbidden to enter the woods behind the house, and she assumes that it's just because of the danger of getting lost or meeting wild animals.
But when Penny runs out of the house, upset because Ann Marie has given her room to a guest whose sobbing disturbs the other guests she learns otherwise.
Deirdre is only the latest Heroine to monopolize Anne Marie's attention, and the adolescent Penny is frustrated and angry. She rebelliously enters the woods and meets Conor, a knight from Celtic mythology who is searching for his betrothed, who just happens to be the young woman in Penny's room.
At first I wasn't sure what to think of this book, but I found myself reading eagerly to find out what happened next. The story was crazy, and over-the-top, but a lot of fun for someone like me who always wondered what it would be like to meet the heroines of my favourite novels.