Usually in graphic novels, since nearly every page requires a lengthy description, it is better to add the description using alt-text, instead of linking to a long description on a separate xhtml page—that would certainly be a clunky reading experience.
As for multiple panels or frames on a page, the alt-text can note that the page has multiple panels and then describe each one in reading order. For example:
alt=”Multi-panel comic. Panel 1: […] Panel 2: […]” etc.
There is a way to use aria-describedby to provide a longer description for an image as well. However, since aria-describedby does not have wide support across different reading applications, I would stick to the alt-text method.