One reader over at MobileRead Forums noticed something when he took apart the latest Epub he'd bought. It looks like someone at the US branch of HC is using Sigil to create and work on Epub ebook files. The title in question is The Washington Trilogy, a political thriller.
Now, he had to take apart the ebook because while there's no obvious mention of Sigil, it is referenced in the metadata. Curiously enough, there's no other tools mentioned, not InDesign or anything. That leads me to think that Sigil was used to create the ebook as well as edit it.
I can understand why. Sigil is a pretty nice tool for both editing Epub and converting to it. That's why I use the app, and that's probably why the reader was using it. I'm sure he wanted to fix the layout so it looked best on his preferred ereader.