What goes wrong? Did you manage to embed these fonts? Are they used in your book or is the system font still used? And have you changed your css-file correctly? I have the impression that the iRiver is very strict in this respect and ignores many things if the css has some problems.
Maybe this thread helps you a little bit, especially post 11 and the many links to other threads:
https://www.mobileread.com/forums/sho...d.php?t=153778