Whether you feast on truffles or Cheetos, the end result is the same. Defecation is the great leveler. There’s nothing more democratic or unpretentious than what happens in the confines of our commodes — as the children’s book reassures us, everybody poops. While we increasingly obsess over what we put in our mouth, we pay little mind to what becomes of our meals once they go down the hatch, at our own peril. Two new books encourage us to recognize ourselves in our most humbling bodily affairs. “People are surprised to learn: They are a big pipe with a little bit around it,” said one …