Of course a book on how to get off a deserted island when stuck.
Or at least a book on how to survive on a deserted island.
Or at least hope to watch old Gilligan's Island reruns. Ahhhhh - I am dating myself. [It is REALLY WEIRD to date yourself. Like, what do you say on the date?? "So, how many siblings do you have?" I know the answer to that and all the "getting to know you" questions].
But of course I would want a Tanach and/or a Shas. But if I had to choose a sefer of a rishon, the Hilchos Teshuva of the Rambam would be IRRESISTIBLE!!! [And in the same volume there are other goodies too].
LEARN IT AND LIVE FOREVER STARTING TODAY!!!!