For obvious reasons, I didn't feel like reading an already-digitized book while I was in the British Library--the whole point being to go to the British Library to read otherwise-inaccessible books--so I Googled each title before calling it up. (Yes, this took a while.) This led to a somewhat unexpected chain of events:
1) In the USA, I had already searched a bunch of titles and found no sign that they enjoyed a free existence.
2) In the UK, I searched some of these titles again, and...there they were in GoogleBooks. Say what?
3) Back in the USA, on a lark I began looking up some books I had found available on Amazon in facsimile form, and which had not cropped up in GoogleBooks (or archive.org, or HathiTrust) when I searched in the UK. And there they are.
Bear in mind that most of these books bring up only one or two pages of results--we're not talking about combing through thousands, hundreds, or even dozens of hits. I didn't overlook them; they just weren't there. It's possible that Google "learned" my preferences, but surely it could have brought up an item it hosts the first time around?
Moreover, I've also found that some books cannot be searched via GoogleBooks at all; you have to use the regular Google search function. To take a minor example: GoogleBooks pulls up volumes one and three of Mrs. O'Shea Dillon's triple-decker Dark Rosaleen. Where's volume 2? (You know, the middle of the book?) It turns out you can get it on Google Play or at the French GoogleBooks--but not at the American or English GoogleBooks sites!