OK, so I'm replying to my own post--but I simply had to say how much I'm enjoying these letters. I'm savoring them, so I'm only up to early 1917, currently reading a couple of letters Cather wrote to her mother. These letters strike me as dutiful and sweet, newsy letters about things she thought would interest her mother. Cather was a good if unconventional daughter, and I don't think she ever gave herself a break about that. As a daughter of a very "eccentric" 88-year-old mother, I have some sympathy. 
My favorite review of this volume, so far, is by Jennifer Howard at The Chronicle Review, someone who actually did her homework and interviewed people who had knowledge of the project. 
What have I found interesting so far? Well, some of my favorite letters have been the ones that Cather wrote to Elizabeth ("Elsie") Singer Sergeant (c. 1915 or so, before and after). I was struck by how Cather frequently mentioned Isabelle McClung in these letters, yet she NEVER mentions Edith Lewis. Just an observation--interesting, I thought. Does anyone have any insight into that?
ESS was a friend of Amy Lowell, and Cather was an "enemy" of Amy Lowell. I once thought it would be interesting to find the letters between ESS and Lowell and see if Cather is mentioned in any of them--but it's a project I never got around to. I also wish that someone might do a volume of letters between ESS and Cather, putting them in context, etc. Is there anyone out there working on ESS? She is the older sister of Katharine Sergeant Angell White, fiction editor for the New Yorker from about 1925 to 1960, so that fact adds an interesting dimension. Anyone interested in magazine publishing in the 1920s-1930s could have some fun with this.