Tested and working:
How to edit iBooks meta data under Mac OS X Mavericks
First of all, I GOT THIS FROM ANOTHER SOURCE. I take no credit nor responsibility.
IT WORKS
Edit metadata with Calibre for Mac
Although edit metadata with iBooks is impossible, it doesn't mean this is the end of the story. With another software, Calibre for Mac, we can edit book's metadata then load the book into iBooks for Mac app.
First we import the book into Calibre and delete it from iBooks
Right click on the book title, choose "Edit metadata", then "Edit metadata individually".
In this window we can customize almost all the important metadata, including adding a cover to the book
After you finish the editing, click "OK" to confirm.
But job is not finished yet, in fact the editing is only done in Calibre's library. To display the edited book in iBooks for Mac, we need add the editing into the book file.
No matter if we are adding cover or modifying other metadata like title, author & publisher, genre, etc. , converting the book to EPUB again is necessary, only in this way, we can completely edit the metadata inside the book, not only in Calibre library.
So the last step is to convert the book to EPUB, even this file is already an EPUB file.
Find the converted file in Calibre's library and drag it into iBooks for Mac, now we can see the book displayed in iBooks for Mac with all the gorgeous editing present.
Using the same method we can also add or modify other metadata like title, author & publisher, genre, etc. on Mac OS X Mavericks even the iBooks for Mac won't permit us to do so.
I have tried this and it works!!!