@SirYeen - PDF 'books' don't have a designated cover as such, whatever is the first page of the pdf is the cover.
You can use the Embed Metadata tool to embed the metadata into a PDF. I will note that I've had mixed results with this, however I've been able overcome any glitches by clearing the existing metadata from the PDF with Trackers XChange Viewer and re-doing the Embed Metadata.
To add/change the cover I think you'll need to use some sort of PDF editor. I've done it with Tracker's xChange Viewer - by pasting an image at the front of the book from the clipboard.
BR
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