I just got the 5" Pocket Pro this week. I haven't read throlugh all of the free books yet. I tried different fonts. My older eyes need fonts that are thicker, or with a Bold option. Thin lettering is hard for me to read. Laser retina surgery tore up some of the rods and cones in my eyes, so bold fonts are easier to read. I have good eyesight, its just low-light or thin letteting is difficult to see well.
I'll check some of the fonts, and see if particular txt ebooks are easier for me to read than other fonts. I'll post to let you know.