One thing to keep in mind here is that the Internet is absolutely the worst way to search for books that haven’t been digitized.
Instead, you may do well to find a room of books somewhere that seems likely not to have been scanned.
If you’re totally stumped, ask around about what other people are finding. There are a lot of different strategies for this.
In my house, for example, I’ve turned up the following books that don’t seem to have been digitized (I haven’t done a totally exhaustive search yet):
A 1904 “Fullstandig Varlds-Atlas” published by the Svenska Amerikanska Post in Minneapolis; probably something given away to customers by an ethnic newspaper and so not collected by libraries.
The Telberg Book company’s 1956 English translation to “Atlas Mira”; a Mimeographed set of pages bound weirdly together.
When I Wished I was Here, 2007, by Derek McGee – a short book/pamphlet published by a kind of hipster press in Brooklyn.
Is it Time for the Party? By Julia Summers. Hallmark Children’s Editions, c. 1970. Out-of-print children’s book with a plastic clock bvuilt into it.