Agartha’s Castaway, by Chrissy Peebles

Genres for this novelette series: fantasy/young adult/adventure/sci-fi.
See the book trailer below for “The Trapped in the Hollow Earth Novelette Series”

*** The dream vacation that might just turn deadly…
Seventeen-year-old Casey Smith can’t wait to embark on her summer vacation, not least because she’s finally shed her tomboy image, and now heartthrob Mike seems interested. What starts out great, with lots of flirting, taking her a step closer to winning her crush’s heart, soon turns into her worst nightmare.
Tossed out to sea during a freak storm, Casey is washed up on what she presumes to be a mysterious tropical island that’s on no map she’s ever seen.