I think he means like the Book of Jasher (for example). We can reasonably conclude that there was a "real" Book of Jasher in antiquity because the author of the Deuteronomist History referred to it. Even if you want to say that the Bible isn't "inspired" it's still reasonable to conclude that this other book still existed at the time it was reported to have.
Subsequently, the book was lost through some means as Prax describes - sands of time or deliberate destruction, who knows. Then suddenly I find that I can get a "Book of Jasher"
on ebay; or better yet, you can get it for me as a gift.
Now, where did this thing come from? It is obviously a copy of a copy and so forth. But is it THE Book of Jasher? No. Its pedigree has already been determined to be fraudulant, probably by folks who actually ponied up and bought copies of the scrolls on to be disappointed later.
There was an "original" book and certainly copies and copies of those copies; but that whole family has been lost.