Book out of Elsewhere

It used to be called 'speculative non-fiction', but this name seems to have turned into something else already, like just about any genre. What I am fascinated by are books that are entirely true to their imaginary origins, containing no clue as to their real world beginnings outside of what you might find with forensic means. These books lie completely in their own imaginary world as if they were plucked uut of an alternate universe, or another time, and dropped into your hands. I have ended up accumulating a number of books like this. Lets hope this hobby stays within reasonable financial constraints.