Ionic seems to be based on the older Dorian epub reader code, according to Nokialino:
One downside is that (there's not) the slightest hint of animation that simulates the real page.
epub format ebooks can be sourced from many sites, including epubbooks.com and Project Gutenberg. Note that DRMed (commercial) books are not supported. As Nokialino notes, "the numbering (0.6.0) suggests that it is still a beta", so there are improvements yet to come.
You can find Ionic for free on the Nokia Store.