Sadly I am not much of a book reader in the main, howere I though I would kick of the replies to this subject, in my own way... Even though I am sure these were not what you meant for choices of book
(1) How to build a yacht for dummies!
I'm thinking this might come in handy for a trip to the shops, even if I will want to reclaim my dessert island ;)
(2) 101 ways to enjoy coconuts and fish
I'm sure even then, the island diet might get a little old pretty quick
(3) Castaway - Lucy Irvine
Having watch the movie years ago, I would really feel the book might well connect better in this situation. However I do seem to remember thinking what an awfully strange situation and complicated characters these two were.
(4) Tropical Diseases - A practical guide
Although I must admit we did have an elderly family member who had a similar tome taken away for the benefit of her G.P. This however would probably be much more practical and useful given the situation.
(5) Oddysey - Homer (Not Homer's Odyssey by the Simpsons)
Obviously this book has been an inspiration to many and what better place to read about the long journey home than when stranded on a tropical island ;)