One way to create an Excel Catalog of ebooks it to use Calibre. Import all your ebooks into Calibre, select all the ebooks and under the Convert books icon select "create a catalog". If you include the cover as a column in the spreadsheet what you get is the path to the cover JPG that Calibre produces.
Once you see what Calibre can do, you may not need a spreadsheet. One thing it can do is get better cover images for ebooks that lack them (under Edit Metadata).
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