Q. When citing a book in a bibliography, endnotes, etc., one does not include the name of the library that holds the volume consulted. Why, then, must we continue to include the URL of books we’ve consulted online that have been scanned by Google Books, HathiTrust, or the Internet Archive, to name a few such providers? Isn’t the internet as common a place a researcher would go to find a book these days as is a library or bookstore? Why is it necessary any longer to give internet sources “credit” for “possessing” a copy of a book when physical holders have always gone “uncredited”?
A. Do it for your readers. Most of them will have access to the three databases you mention. And each of those databases provides full access to many books in the public domain, which in the US has long included works published before 1923 (see table 4.1 in CMOS for a summary of the rules; note that, as of January 1, 2019, according to the ninety-five-year rule, works published in 1923 have also entered the public domain, in a process that will be repeated at the beginning of each new year). Providing a URL for one of those books is as good as handing it to your readers to examine for themselves. Let’s say you cite the first edition of Jane Austen’s Sense and Sensibility, published anonymously in 1811. Readers would have to do some digging to find that edition without a link. So why not provide one?
Austen, Jane [as “A Lady”]. Sense and Sensibility. 3 vols. London, 1811. https://archive.org/details/sensesensibility131aust/.
That way readers will see what you see, and if you publish your work, you’ll be prepared to link to the source however you want. For example,
Austen, Jane [as “A Lady”]. Sense and Sensibility. 3 vols. London, 1811. Internet Archive.
A link to a database has some additional advantages. For example, readers will learn from the Internet Archive’s record for Sense and Sensibility that it was contributed by Duke University Libraries. A bit more research will lead you to the physical copy at Duke’s David M. Rubenstein Rare Book & Manuscript Library. But you don’t have to add that to your source citation.