The story is about a group of children who are abducted by pirates, with brutal, comic, and consistently unsettling results. It was a favorite of Flannery O'Connor and I can see why: it exhibits a startling mastery of tone shift, moving from the ridiculous to the violent to the wry to the morally fraught within a page, sometimes even a paragraph or sentence.
The novel is often cited as a precursor of Lord of the Flies, perhaps because the children escaped from civilization's bonds do appalling and heartless things together with cute and kidlike things. I imagine a book club reading it would soon get to arguing over whether children are not at all like this, or are exactly like this.