I read ebooks on a Symbian phone (Nokia E61) in a variety of formats. The software available includes Mobipocket Reader, eReader, Bubue Books (for non-DRM epub files), QReader (for Palm DOC files) and iSilo. TXT files can be read and edited with LightNotePad while Nokia provide a free PDF viewer with the phone.
There is no way of reading Kindle ebooks or DRM'd epub files, but I do not find this a problem because I can read the epub books on a Sony Reader.
For more information on Symbian a very good website is www.allaboutsymbian.com