The Different Girl, by Gordon Dahlquist

Four girls live on a desert island with their two guardians.  Each day, they learn new things in their school about the world and their abilities.  Each night, their guardians put them to sleep.  Completely inseparable, the four girls communicate seamlessly with each other, sharing thoughts and completing each others' sentences.   It all seems normal until the day that a fierce storm comes and suddenly a new -- and very different -- girl is washed to the shore.

But it isn't just that the girl is different.  The island itself is changing and danger seems to be approaching quickly.  The guardians begin warning the girls that their very survival may be at risk.  And as previously unimagined dangers close in on the four girls, they come to rely on the new arrival to guide them.

An unusual, striking, and original story.  The mystery of the girls themselves and how they came to live on the island is very slowly unraveled (but is also a process which is never completed).  Instead, we are left with a story with many things unexplained.  The loose ends offer up a choice of interpretations to the reader.  Yet, given the situation, it is natural that a vast majority of the story's action occurs off-page and is beyond the ability of the narrator to explain to us.  Some readers may find that maddening, but I think it provides a fascinating dynamic for the reader to absorb, leaving us sometimes as bewildered as the four girls themselves.