There appear to be a large number of rules of thumb for histogram bin size and kernel selection for density plots. Are histograms and/or density plots really the best visualization for a single continuous variable? Are there other desirable options? Are there any settled best practices for visualizing continuous data that has no known distribution? Is there a good citable comprehensive reference source in the methods available and their advantages/disadvantages?
You ask for a citable reference, I would go for one of William Cleveland's book Visualizing Data, also see his author page. See also this earlier questions: https://www.amazon.com/William-S.-Cleveland/e/B000AP9IPS/ref=dp_byline_cont_book_1 and What's a good book or reference for data visualization?.