This is most likely due to the default font used by mobile Adobe Digital Editions. Almost all vendors are using Adobe's provided font, and it does not include many glyphs.
On some devices it is possible to provide your own fonts for mobile ADE (with a small addition to each ePub's CSS), but I don't recall reading about anyone doing this for the iRiver. The other option is to embed a font in the ePub. For a recent discussion of this, see
Esperanto characters?