When I was working at the University of Utah Library a million years ago (maybe only half a million), my past-time on break was to walk the stacks, it was exercise and I also became acquainted with all of the different sections of that huge library. I would also find books of interest from time to time. Whenever I go to a library, I essentially do the same thing, walk up and down the shelves looking for relevant material. I have learned that no index (card catalog) is perfect and there is no substitute for taking a look for yourself as to what might be available.
The current equivalent for walking the shelves is using Google Books, WorldCat and Archive.org to look for books that might be helpful to my research. But I do miss the library.
Here is an example of an interconnected search on Google/WorldCat/Archive.org:
After opening Google Books, I did a search for "tanner genealogy." I got back 13,300 results which gives me plenty to work with. I did another search with a place name, "New York." I immediately found a reference to a book in the Bulletin of the New York Public Library, Volume 16:
Google Books has the entire book digitized in full view. The citation to the book is Tanner, George C. William Tanner of North Kingstown, Rhode Island, and His Descendants. [Minneapolis, Minn.]: Pub. by the author, 1905. Let's suppose I wanted to find the book in a library (despite the fact that the book is completely digitized online). I can click on the "Find in a library" button in Google Books and Google will take me to WorldCat.org, where I find that none of the close-by libraries have the book.
At this point, if I wanted to pursue the matter, I could order the book through Interlibrary Loan from my local Mesa Public Library. But I want to see if the book is located anywhere else on the Internet, so I click on the link to "View all editions and formats."
I find several online copies of the book, some in subscription websites, I choose the Archive.org version which gives me the option of reading online, downloading a PDF, a B/W PDF, an ePUB version, a Kindle version, a Daisy version, a Full Text version, or a DiVu version or having the entire book in HTTP.
I realize that you may not have the same luck in finding a book about a remote ancestor so easily, but it doesn't hurt to look and search on a variety of names and locations in all of these resources.