The book of speculation

A young librarian named Simon Watson lives in Long Island Sound where his mother, a circus mermaid who was paid to hold her breath, drowned. Now his younger sister, Enola, has also ran off to join the circus. Then Simon receives a book in the mail from the 1700s recording the drowning death of a circus mermaid, and generations of mermaids in Simon's family, always on July 24. With that date only weeks away, Simon races to save his sister from the family curse.