A beta version of eReader.com's eBook software is now available for S60 3rd Edition. eReader is a long standing eBook store which provides some of the current best sellers in eBook form, protected via its own form of DRM. They provides a version of their reader for multiple platforms. However a few years ago they seemed to lose interest in support Symbian OS handsets, but under the new ownership of Fictionwise this seems to be changing.
The S60 3rd Edition version of eReader will be most attractive for those who have already bought books using eReader. If you're looking at buying an eBook for the first time you may also want to investigate the Amazon backed Mobipocket which provides both reading software and eBook store.
Both Mobipocket and eReader povide similar features in their software (full screen mode, the ability too look up words, bookmarks, autoscroll, font and text settings). Both can also be used to read standard .pdb file (non-DRM) which is useful for reading public domain content from sites such as Memoware.
Mobipocket is currently the more mature reader (and is also available for UIQ handsets), but hopefully the eReader software will progress during the course of its beta.
eReader is owned by Fictionwise, another eBook store, which has the notable positive feature of providing many of its books in multiple formats including both eReader and Mobipocket. Books bought from Fictionwise are therefore less susceptible to being left behind by changes in DRM.