So I was going through my vaults to gather goods to take to my end base when i came across this plain enchanted book. I can't find anything about it by just typing in plain enchanted book.
That is an interesting find there. I might take a guess and maybe say 2500r for the book I don't know honestly lol
I found a reference to a Bukkit bug here that might be relevant. You'll find it almost all the way down the page with the heading: "Bug is not a Bug" and reads, "One notable bug is that when a player logs out of an SMP server with an Enchanted Book in their inventory, the book loses its enchantment name and becomes a useless "Enchanted Book" on re-entering the server.". I didn't dig any further though. Perhaps Aikar could tell you whether it was ever an issue here that may have created it. If it was created through a bug, perhaps someone should dispose of it.