Calibre, the leading ebook management/reading/library/conversion app, is now also an ebook editor. This new feature was added in a weekly update about 2 weeks ago, but it only came to my attention this week.
While calibre has long enabled users to edit a book's cover image and metadata (title, author, etc), it wasn't until recently that users had the option of editing the contents of the book itself. And as you can see in the screenshot below, calibre can now edit any DRM-free Epub or Kindle (AZW3) ebook:
Users can edit the HTML source, delete and add files, swap out the cover image, and then run a check to see if any of the files in the ebook aren't mentioned in the spine. It's not yet possible to work with the fonts or draw a cover image, but users can edit the TOC as well as the manifest and other ancillary files found in an ebook.
I haven't had a chance (or a need, for that matter) to use the editor myself, so I would love to read what you think about it. The editor is included in the latest update on the calibre website. After you install it, you can open the editor by right-clicking on a title and selecting the edit option.
Initially launched in October 2006 as libprs500, Calibre has become the leading open source ebook management tool - and more. This app, which reached 1.0 in August 2013, now supports 3.6 million users in 236 countries who use calibre to both convert ebooks to a wide variety of formats and transfer the ebooks to an unbelievable number of ereaders and ebooks. It now works with most Android tablets and smartphones, iOS devices, and more ereaders than you can even conceive of (seriously, there have been ereader which i first discovered when they were mentioned in the calibre changelog).