Sigil is an Open Source eBook editor featuring a WYSIWYG panel. That means that What You See Is What You Get. It works very similar to Microsfot Office programs, so you'll get used to it quickly. Nowadays Sigil can import files formatted as TXT, HTML and EPUB and export files as EPUB and SGF (Sigil native format). Although the program is in a very primary phase, it is very easy to use and offers good possibilities to eBook owners.
