For those electronic books, which support function of loading of additional fonts of TTF format (True Tupe Fonts), we have created a special set from eight various bold fonts. (e-ink-reader.ru)
You can add additional fonts by placing TTF files in the Fonts directory of your device (according to the Cool Reader manual); similarly, you can add your own background images if none of the 30 provided with the program satisfy you. (computerworld.com)