Yes, there is Mobipocket...
However, to have an ebook reader for the only open code file format - EPUB - would be really very cool.
An epub ebook is a zip package with XML and XHTML 1.1 ...
Surely not anything too complicated.
You can read epub ebooks thru web service such as bookworm.oreilly.com
but a standalone reader for S40 and S60 would be great, I think.
ps: okay, this one got to the wrong place. my bad, sorry.