Please note: although pepak's
hack has the great advantage of working for most epubs without editing them, it does require flashing the firmware using igorsk's universal flasher, which AFAIK does not work on the 350/650 (because it relies on autorunning from the SD Card, which Sony seem to have disabled in the later readers).
This differs to jswinden's
method, which has the advantage of working with the 350/650, but the disadvantage of requiring you to edit the epub (to add the font info, especially the location of the custom fonts on the reader).