Some books do not get a lot of readers; they are not read by many. Sometimes because they are not that good, sometimes because they are not advertised enough, or perhaps for other reasons, they are kind of isolated in the world of readers. What to call such books?
as pointed out, little-read or obscure are obvious choices with little to no bias to them.
If, instead you wanted to connote that a book has not been read by many because it is a failure, you could refer to it as a flop or a nonstarter.
Conversely, you could connote that a good book has not been read much by calling it underrated or underappreciated.
Finally, media which is not necessarily rare or unknown but has a relatively small and fervent following is often said to be a cult classic