Mine is The Body by Bill Bryson. I found it fascinating and some of the facts in it are amazing, my favourite being the following.
’Every time you breathe, you exhale 25 sextillion molecules of oxygen- so many that with a day’s breathing you will in all likelihood inhale at least one molecule from the breaths of every person that has ever lived. And every person who lives from now until the sun burns out will from time to time breathe in a bit of you. At the atomic level we are in a sense eternal’.
I find that fact fascinating and oddly comforting.