Hard to pick the best, and the pleasure of reading them was much greater when I was under 20... but the one I have re-read most times may be _Gods, Graves, Scholars_ by C. W. Ceram. Runners-up are Dostoyevsky's _The Brothers Karamazov_, _Microbe Hunters_ by Paul de Kruif, _The Mysterious Island_ by Jules Verne, _Solaris_ by Stanislaw Lem, and several others that don't occur to me now...