You might think that an unpublished book was one that had never been presented to the public. And if so you’d be right, for the Concise Oxford English Dictionary defines the word as:
adj. (of a work) not published. > (of an author) having no writings published.
and the Oxford English Dictionary has:
1.a Not made generally known or accessible, esp. in print.
1.b Of an author: having had no writings published.
2 Not divulged or disclosed.
Neither dictionary offers any other meaning. However, Amazon (and possibly other publishers) have given it a new meaning: removal from sale of a book that has already been published. That is to say, first it is published and then it is unpublished.
It occurs to me that the prefix un- does have two meanings. This use in unpublished is the same as unplugged and untied, where something that has been done in undone and is different from unknown and unwritten, where something has not yet been done.