Agreed. The higher costs of the hardcover books, is my guess, made them commercially viable at much lower printings - probably up to today.
One thing that in earlier times (by this I mean pre-WW II, and early postwar period too, probably) was a source of reading material for the general public were (perhaps still are) the public libraries. The NY Public Library, a really big one, was founded in 1895, as I just checked. While I rarely if ever borrowed books from public libraries near where I lived, I certainly spent a lot of time in my high school library, and read quite a few books borrowed from it.
So readership goes beyond those who actually buy books. But in the time before the advent of electronic cataloging, getting those kinds of statistics must have been a chore, not to say almost impossible.
The popularity and influence of the authors that you mention would have been with the readership, no question. But the only thing quantifiable was actual sales of books. And those cannot have amounted to "mass markets" - which you do enclose in double quotation marks, and rightly so. A mass in relation to the small market of (hardcover) book buyers. But before the widespread advent of paperbacks, operating on much slimmer profit margins and thus dependent on big sales, I don't see there being anything that can truly be called a mass market. Part of the efforts by the paperback publishers, as in any comparable "industry", is to lower costs. Which is why my mid-1980's LoTR paperbacks are quite tattered ... those of my early JRRT books that are not have simply not been read as much. Practically all of my more recent acquisitions have been hardbacks, or at least trade paperbacks.