To me “Echo McCool, Outlaw Through Time” by Roger K. Driscoll, while listed as a YA, has equal appeal to both the YA and adult reader.
Echo McCool is a half-blooded dryad. At the beginning of the book, she is shot by a poisoned arrow and, to escape the villian and survive, she jumps into an old, large, hollow oak. Too tall for her to get out, she enters the big sleep–better labeled the long sleep.
Enter 12-year-old Jason Fleeting who is dying in a hospital. He meets Echo’s mother, a guardian of the gateway. She tells him she can heal Jason if he will rescue Echo from her tree before some loggers saw her in half. Jason works his own bargain, he wants to find his sister–who was kidnapped by their mother’s killer.
Bargain stuck, Jason runs away from a children’s home. Together, he and Echo set out to solve the murder of Jason’s mother, rescue his kidnapped sister Lauren, and restore an inheritance to a new friend.
This is a fast read and I look forward to Echo’s, Jason’s, and Lauren’s next adventure.