Flu* by amazing science writer Gina Kolata, is currently available cheap in kindle form. This book takes you up to a critical point in time in the understanding of the influenza pandemic of 1918. A lot of things were discovered after Gina’s book came out, so it is admittedly not current, but it is nonetheless a classic.
Also on sale, the very recently released Pump* by Bill Schutt (see also our Ikonoklast interview)