The Secret of Bald Rock Island by Charles A. Salter

Ten-year-old Kelcie Oglethorpe's fisherman father disappeared at sea years ago.  Since then, Kelcie has befriended Bald Rock Island's beloved Mr. Bartleby, a kindly man devoted to the village and its children.  He often warns of a mysterious creature he once saw on the island's bald dome which towers a hundred feet above the sea, yet is hollow inside and connected to sea life by underground channels.

Kelcie decides to help Mr. Bartleby solve the island's two enduring mysteries--what happened to her father, and what the sea creature might be.  She learns that Mr. Bartleby was her father's lifelong best friend.  During that terrible storm off the coast of Maine, Mr. Bartleby risked his own life to do everything possible to save Kelcie's dad, but no sign was ever found.

Kelcie criss-crosses the island in search of clues and thinks she knows where she can find the ultimate answers.  She resolves to explore the forbidden area of the island, and as she puts the final pieces of the puzzle together, a new storm arises.  (taken from the back of the book)

I was pleasantly surprised by this book.  It's exactly the sort of thing I would have devoured in third grade.  It has a wonderful amount of suspense and a nice brisk pace.  Even though there are some adult themes, they're dealt with in a child-friendly way.

Salter's storytelling reminds me of the way children's books were written in the 1950's.  It has that wholesome feel to it.  I would be ecstatic to recommend this to any child that loves adventure or mystery.