The versatile EPUB editor, Sigil 0.5.3, available via PPA
Sigil is definitely one of the most versatile EPUB editors with plenty of functionalities, like full UTF-8 support, full EPUB 2 spec support, multiple views (split, book and code view), metadata editor with more than 200 entries, support for spell checking, SVG support, WYSIWYG editing in Book View, table of contents generator with multi-level heading support, support to import EPUB and HTML files (and images, stylesheets, fonts) and more.
Sigil can be manually installed (by downloading the Linux package from its official page), although it seems that on Ubuntu there are issues (like for instance, Sigil can only be launched by always double-clicking the executable from its installation folder).
Nevertheless, the latest Sigil version, 0.5.3 (a workable version), is available via a PPA (Maverick, Natty, Oneiric, Precise)
sudo add-apt-repository ppa:rgibert/ebook
sudo apt-get update
sudo apt-get install sigil