Two new books belong in your bathroom library, one by the inimitable Mary Roach, of “Stiff” fame (see first answer^^^). This new one is called “Gulp” and is “an odyssey through the digestive system by Mary Roach.
“Roach guides the reader gently down the upper reaches of the alimentary canal like a trustworthy gondolier before hitting the bawdy, chaotic rapids nearer the end of the journey.”
And in the same hilarious review
David Waltner-Toews’ aptly titled “The Origin of Feces”; “Once we are safely out of the intestines, back into the light of day at the other end, David Waltner-Toews picks up the thread with his impassioned treatise on the long, strange, even transcendent afterlife of poop in The Origin of Feces, a book whose cover is guaranteed to make you few friends at the coffee shop.”
Disclaimer: @syz first suggested Roach’s books to me.