Web content server error: 404 Not Found (The path '/browse/category/' was not found.)
it is obvious that 404 error is shown when link is pointing to nonexisting page.
When there is defined book without attached files, clicking on such book cover in its web page record provided by web content server (sharing over the net) it lead to error "404 Not Found (The path '/browse/category/' was not found.)"
Clicking to book cover in web page, it usually point to first book file.
Maybe it would be better to not make covers clickable for books without file, or display some message page instead of error 404. For example message "There is no ebook file for this book."