After the iPad came out, everyone (myself included) wanted to know how well it would handle epub. As it turns out, not great. No embedded font support? Bad Apple, bad!
Anyway, the iBooks application for reading epub books on the iPad comes with this large shelf view where all the books display their covers. Shiiiiny. But as a content producer, you have to explicitly tell the iPad which image in the epub archive is the cover.
There was an interesting discussion about this on MobileRead. The idea to take away is that you need a special meta tag in the metadata section of your OPF file. Something like this:
<meta name="cover" content="coverID" />
The “coverID” needs to be the ID of the cover image in the manifest section.
Until now, you had to save your epub with Sigil, extract the epub, edit the OPF, add this meta tag and then zip everything up properly (mimetype file first with no compression etc.). Oh and you could never open that epub again with Sigil since it would remove that special meta tag.
A horrible PITA, wasn’t it?
Well that’s fixed now. The next version of Sigil will have a right-click context menu for images with which you’ll be able to mark an image as a cover. Done. Sigil does everything else required for you.
Along with this “Add Semantics” menu come new entries for the HTML resources. For those, you can now add the <guide> element semantic information. So if you mark one HTML file as, say, a “Title Page”, Sigil will add this information to the <guide> element in the OPF.
Naturally, this feature would be useless if Sigil wouldn’t preserve all of this information after opening an epub that already has it. So it does that now too: all information from the <guide> element is now preserved, including custom values for the “title” attribute (even though you can’t see that value in Sigil, it’s stored). The special meta tag identifying the cover image is read and that information is preserved, too.
Being smart about it
Sigil always does its best to help you out. Well, it at least tries :).
I’ve added heuristics to Sigil that will mark the appropriate image as the cover if you don’t do it yourself. If the first HTML file in the reading order is “very small” and has only one image in it, that image will be selected as the cover.
So if you follow best practices, Sigil helps you out. Still, mark it by hand if you can. You will always know better than the machine.
Things to note…
While the OPF spec technically does allow you to, for instance, specify several HTML files as the title page in the <guide> element (for God knows what reason…), Sigil stops you from doing this. It allows you to set only one instance of one <reference> type per book. So if one file is set as the title page, setting another file as the title will unmark the last one.
The exception are loaded epub files. If your loaded file specifies several HTML files as, for instance, the preface, then all of those are still marked as such in Sigil after they’re loaded. While I personally think you should never use more than one reference type instance per book, if you did this to one of your books before opening it in Sigil, that information will remain. Sigil won’t step on your toes here.
Finally, here are two images showing off the new menu. The file loaded is Three Men in a Boat. If you don’t want to wait for the next release, you can build from repo source and start using this right now.
 Sigil looks for a normal <img> tag or an SVG <image> one.
 It’s a terrible idea, and it would probably wreak havoc on unsuspecting Reading Systems. The spec actually doesn’t explicitly allow it, it just doesn’t talk about this possibility at all. I’m still not convinced that the spec writers didn’t just forget to forbid this behavior.
So Sigil won’t let you do this.
 Which will be RC1. No more betas! :)