Similar to what Bandcamp does with album covers.
When I click on a cover, whether the main one or an edition's, open a lightbox with the cover in full resolution.

Several reasons why:

• Some covers are very pretty and I want to admire them up close
• Helpful for librarians: download the cover to assign it to another edition that uses the same cover; read the author's, translator's or publisher's name on the cover to use in filling data... those details are often too hard to make out in a small thumbnail but very clear on the full resolution image, if you have access to it
• Helpful for any kind of note takers, journalers, dairy writers, scrapbookers... Who may want to get the full resolution cover for their daily notes, book reading journals, personal archives or whatever.
• Why ask for the highest resolution possible if we're never going to display it anywhere ;)