See, this is why I never travel to desert islands. They only seem to allow one CD, one book, one movie...
I suppose I might go with something by Stephen Baxter, as I always have to re-read his stuff just to figure out what's going on... Nah, maybe not. Too intellectual, in the long run. I don't know if I'd be as interested by the 20th read-through.
I suppose I'd either bring Lovecraft's body of work -- creepy, yes, but fun to re-read.
And if I absolutely have to pick ONE book -- no more hedging -- I'd go with Childhood's End by Arthur C. Clarke. At least it gives the reader lots to think about, making further reading a worthwhile endeavor.