1) You're alone on an island for who knows how long. What three books would you want to read?
2) We'll assume that somehow you can survive well enough, so you need not have a copy of "How To Survive on An Island" or something of that sort.
3) Of course, just for staying on the island you get a fabulous "Complete Works of Shakespeare" as yours to keep (thank you Mr. Smith!)
4) Also included in this island getaway package is a free Bible or appropriate religious work (Koran, Book of Mormon, etc.)
5) You only get to choose 3 books, so you can not have all of the Harry Potter series, for instance, or all of the Bobbsey Twins, The Mortal Instruments, The Eragon Saga, The Enemy Series, Sue Grafton's Alphabet Murders, A Series of Unfortunate Events, or 57 Varieties of Nancy Drew.