Erik Kwakkel over at medievalbooks.nl gives a great rundown of early advertising for books, including examples window ads, publisher spam within manuscripts, and posted flyers.

While this does not give us a clear picture as to how common advertisments were in medieval times, it does give us a clearer picture as to what form they took.

Window ads would work as a preview to the actual book or manuscript being sold, with grand examples of script and color, including large gilded letters.

Spam, for lack of a better term, could be found in the back of books extolling the quality of the book just read with directions on how a reader could find the maker and buy more items from them.

Lastly, posted flyers would offer a sample of the printing techniques that could be found in the books being offered for sale.

Kwakkel goes more in depth with the early days of commercial book production in a lecture he has posted to YouTube.

Source: http://medievalbooks.nl/2014/12/05/medieval-spam-the-oldest-advertisements-for-books/