How often do people write about proportional representation?

This graph from Google shows how frequently “proportional representation” was mentioned in the vast Google Books archive.

As you can see, the proportion of books which mentioned PR grew sharply in the late nineteenth and early twentieth centuries, reaching a peak in the inter-war years (when a minority Labour Government got an alternative vote bill through the Commons but it then stalled in the Lords and was abandoned when the government fell):

An excellent book on the early history of proportional representation in the UK is Jenifer Hart’s Proportional Representation.