I think it would be great to have a couple of drinks with Mary Roach. The conversation would always be entertaining. However, for a good overview of the human digestive system, I would suggest Gut : the inside story of our body's most underrated organ by Giulia Enders. It's accessible & informative.
I almost grabbed Gut the other day at the bookstore, but I had this one in progress and a good sized stack already in hand, lol. But I did buy a copy a few months ago for the BFF and she said it was good too. Likely I'll pick it up now next time I'm at the bookshop.
Ha! I'm glad! It's always difficult when a group read goes wonky, but our group so far has been great - nobody has been snippy about the differences of opinion in the group, and the discussion and reviews have been really good. It's a bonus that we've managed not only to entertain ourselves, but others too. :D