I could be all sneaky and say that my desert island book is a solar charger, so I could take my kindle library. But that would just be annoying.
There aren't very many books that I think I could stand re-reading forever. I'm going to cheat a tiny bit and select two.
The first is a leather bound edition of all seven of Austen's novels. Jane is endlessly entertaining.
The second is the single volume edition of Tolkien's Lord of the Rings. It is hard for me to imagine that I could tire of Middle Earth.