One method can be to use aria-describedby to add a longer description for the cover. This method is used often in fixed-layout EPUBs for graphic novels or picture books.
An example is:
<img alt=”Cover: Book Title by Book Author.” src=”images/cover.jpg” epub:type=”cover” role=”doc-cover” aria-describedby=”cover-alt”/>
<div class=”hidden” id=”cover-alt”>Long description of cover.</div>