This is an important question in the context of this project, but it will be one that I have come to decide is impossible to answer. Unfortunately, and (perhaps, for some) frustratingly so.

One important tool (or so I thought originally) to try and answer this question is the ten-page “Timeline of Books on Usage” in Garner’s Modern American Usage (3rd ed., 2009). But the problem with the list is that everything is lumped together indiscriminately: “dictionaries of usage … discursive treatments of the subject … [books with] usage glossaries within them” (p. 925). Bryan Garner already very helpfully answered my question on whether English As She Is Taught, by Caroline LeRow (New York, 1887) is a usage guide or not (it isn’t, he tweeted: many thanks for the quick answer, Bryan!). But surely I can’t ask him to help me classify the items on his list into usage guides and studies on usage!

I do think this would be a really good idea though for a next edition of the book, for which it is probably about time, as doing a little arithmetic suggests – 1st edition: 1998, 2nd edition: 2003; 3rd edition: 2009; 4th ed.: 2016?.