I'm not sure what the default for LRF books is, but the fonts that come with the newer Sony Readers are Swis721 BT (sans serif), Dutch801 Rm BT (serif), and Courier10 BT (monotype), with filenames tt0003m_.ttf, tt0011m_.ttf, tt0419m_.ttf, respectively.
For ePubs, Sony books seem to use Swis721 BT (sans serif) for titles and headings and Dutch801 Rm BT (serif) for the main text. Unfortunately Dutch801 Rm BT is a very light font and it doesn't display well.
You can add more fonts to your Sony Reader and reference them in the CSS. I personally prefer a sans serif font for reading on an electronic reader, so I've added Arial Rounded MT Bold for normal text and Segoe UI Bold for bolded text. Both or heavier fonts than Dutch801 Rm BT and both or sans serif.
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