Plugins support is a revision one at this point. We have plans to expand it out in the future. The system is designed to be flexible and allow for a lot of new features. Plugins are standalone and don’t hook directly into Sigil in term of UI changes. They’re mainly for advanced processing at this point.
Currently Python 2 and 3 are the only supported languages for Plugins. The system is designed to be able to add other languages in the future quite easily. The decision to focus on Python for this release is due to the amount of existing Python code for ebook manipulation that is currently available.
Along with calibre (which added an Epub editor last December), Sigil is one of two open source and free to use Epub editors which are widely used in digital publishing.
Sigil can load html, an existing Epub file, or text. Users can edit the a book's content and metadata, validate it, and then save it as an Epub file.
Sigil can be downloaded from Github.