When I started Sorry! I thought I'd interview some acknowledged experts on manners. However, my first tentative enquiries yielded no positive responses, and I soon twigged that I would get just as much out of talking to unacknowledged experts - people I met while out shopping or on the train.
Among the most interesting books I came across while researching Sorry! was Lawrence Wright's history of the bath and loo, Clean and Decent. One of my friends was disappointed to discover that this isn't the Lawrence Wright who wrote The Looming Tower (about al-Qaeda and the road to 9/11) and Going Clear (about Scientology). Among my favourite bits in Clean and Decent is the statement that "1870 was the annus mirabilis of the water-closet."