Packing for Mars: The Curious Science of Life in the Void by Mary Roach
It's Mary Roach, so no surprise that her book on space focused on how hard it is to take a crap (in space), how much engineering goes into disposing of crap (in space), and practical uses for crap on a Mars mission (including making it into radiation shielding tiles and reprocessing it into crap burgers). You'd think this book would be NASA's worst nightmare, but it's actually humanizing as well as fascinating. If you're a Roach fan you'll love it. If you haven't had the pleasure then this is a great place to start.