I've never tried setting up my Nokia X6 for ebook reading as have other devices that I prefer to use, but I've found some programs for Symbian phones that support a more recognisable ebook format.
ZXReader - Supports TXT and FB2
Q-Reader - Supports TXT and FB2
Foliant - Supports FB2 and other formats.
If you don't have books in FB2 format, then you could install Calibre and convert them to FB2 from a wide variety of existing formats.