It is a common assumption that bestselling books nowadays are mainly commercial, cheap stuff, whereas popular litterature a few centuries ago would only include masterpieces. My line of thought is that the proportion of bad litterature among the best-sellers was as high as it is today, and many bestsellers will be totally forgotten in a few years, maybe decades, as it happened for the last centuries. Only the masterpieces will resist the passing of time.
So, in which proportion were the bestsellers' shelves in libraries full of bad, long-forgotten litterature ? And what kind of books were they ?
Edit - I would be more specifically interested in answers concerning Europe, or French Litterature, but I'll take whatever you give me, eh.
This question may be better suited to /r/AskLiteraryStudies.
Also, do specify which country (or language) you're asking about - thx!
Robert Darnton addresses many forgotten bestsellers of the Enlightenment--ranging from the classic texts of the philosophes to muckraking journalism to pulp fiction and porn--in The Forbidden Best-Sellers of Pre-Revolutionary France.
For lists of bestsellers in 19th-century England, see Richard D. Altick, The English Common Reader, and if you have JSTOR access, this supplemental article.