Can I add or change the author of a PDF file in iBooks?

I had several PDF files in iTunes that I had added authors to. Then I upgraded to Mavericks and launched iBooks. It moved my books and PDFs from iTunes, but many of my PDFs now show the author as "Unknown Author".

Is there a way to add an author to these files? I cannot "Get Info" on the file in iBooks like I used to be able to do in iTunes.

Note: This question was originally asked about OS X 10.9 Mavericks. Now that 10.10 Yosemite is available, there is a way to do this easily in iBooks.

You can use the Automator workflows. Choosing PDF on the left and then dragging the action "Set PDF metadata" (or something similar, my version is in Italian) on the right part of the Automator window you can put author, title and whatever in one or multiple PDF files.

Issues and workarounds: if you update the metadata directly on your PDFs in the iBooks folder it will not update iBooks, so you won't see any change. I suggest to copy your PDFs on you desktop, erase the PDFs from iBooks, use Automator to add the metadata and then re-import (drag and drop is useful here) the books in your iBooks.
I have just finished updating my files, you can't go wrong ;)

To be clear, Bryan Luby's and ReF's answers are both correct. I wanted to elaborate on ReF's answer for those who may have never used Automator before. This will allow you to change the the data for items in bulk.

Changing the Author

1. Open Automator and in the left-most dialog box select "Finder &
   Folders."
2. In the subtree dialog box just to the right, select "Get Specified Finder Items" and drag this box to the top of the workflow zone.
3. Inside the "Get Specified Finder Items" box select "Add..." and choose the files you wish to modify.
4. In the left-most dialog box select "PDFs"
5. In the subtle dialog box, select and drag "Set PDF Metadata" into the workflow after "Get Specified Finder Items."
6. Inside the "Set PDF Metadata" box, check the Author and type in the Desired name.
7. Click Run, the play button, in the top right corner.
8. After this workflow completes you can add these files to iBooks and the Author field will be correct.

If you have previously added them you will have to delete the ones where the Author didn't show up.

Changing the Title

Unfortunately, iBooks reads the title from the name of the file; rather than, the title contained in the metadata of the pdf itself. If you want to change this is Automator it is possible. The most comprehensive way is as follows:

1. As above, use the "Set PDF Metadata" option to update the metadata title to the title of the book.
2. From the PDFs subtree select the "Rename PDF Documents" widget to the end of the workflow.

This method will only work one file at a time and rename the file for you while updating the metadata. This is a particularly slow way to accomplish this task. It would be faster just to edit the file name directly in finder, or, preferably, use command line arguments and regular expressions to rename the file removing the unwanted parts.

In iBooks 1.1 on Mac OS X 10.10 Yosemite:

1. Change to List view
2. Click once on the entry that you want to edit, and wait a few seconds
3. Click once on the author, title, or whatever you wish to edit to make that part editable
4. Type your changes
5. Click off that line

You will have to edit the metadata for the PDF, but there seems to be no way to do this in iBooks for Mac.

One workaround is to use Preview for this:

1. Double-click on the PDF file in iBooks and the file will launch in Preview.
2. Select File > Print....
3. In the lower-left corner of the drop-down dialog select PDF > Save as PDF.... This will allow you to edit the Title, Author, and Subject PDF metadata.
4. Save the new file to your Desktop or any other location.
5. Open iBooks and delete your old PDF.
6. Now drag and drop the new PDF into iBooks and it should recognize the correct metadata.