I just noticed a small bug in the "Preview" (viewer) function that is available from a comic's download page, that seems to exist for all books.
When you are previewing a book, there are navigation buttons immediately above and below the image: a "full screen" button, a left arrow, the "Page xx of yy" text, a right arrow, and a close [X] button. The left arrow buttons are greyed-out (disabled) when you are viewing page 1 of the book, and the right arrows are supposed to be disabled when you are viewing the last page of the book. However, it appears that only the upper right arrow button behaves this way; the bottom right arrow will be disabled on the second to last page of the book rather than the last page.
It's only a minor annoyance, but I suspect it would be fairly easy to fix.
