Another obscure YA fantasy favorite which I think, unfortunately, was hampered from being more successful by its title, which makes it seems as if it's intended for a much younger audience than it actually is, and also doesn't tell much about the story.
What the story actually is? Part ghost story, part mystery, part Indiana Jones for kids (although published a year before Raiders of the Last Ark came out in theaters). When 13-year-old Liz Dutton's archaeologist father goes missing in South America while looking for a lost Maya city, her fear for him draws the spirit of a distant ancestor (Holly Hobbie) out of the portrait she has haunted for two centuries, and the two girls embark on an adventure to rescue him that takes them from Sturbridge, MA, to the heart of the Guatemalan jungle.
The protagonists are likeable, the dialogue crisp, realistic and at the same time entertaining, and the book is packed with fascinating details about the Maya culture, both ancient and their contemporary descendants. Definitely worth a read if you can get your hands on it: it's unfortunately been out of print for years.