Some pirate-type guys dump you on a deserted island, with enough tools etc. so that you could survive quite happily, living from the land. The rogues had a soft spot somewhere, and used their pod to download three books for you – books in the loosest sense, as the first could be the major work of your belief (bible, Quran etc.); the second, the collected works of one person, be it Shakespeare, Dickens, Darwin, or whoever. This has set the scene. The third book you're allowed is what we're interested in. Choose a book that would keep you going for the rest of your life – either by re-reading it without it flagging, or one that you think would take the foreseeable future to get through - and tell us about it.
To get the ball rolling, my offering is "Gödel, Escher, Bach: an Eternal Golden Braid" by Douglas R. Hofstadter. It's full of puzzles, stories, conundrums, pictures and theories. A long time ago, I lent my copy to a 'mate' who was banged up in Lewes nick, for some naughty affair in Gibraltar. I never got it back (I suspect he traded it for a packet of baccy), but I've just acquired a new copy. It would certainly keep me going for a while.