daniels The original font is Caecilia, and it's still on your device. Simply select the font you wish to use from the settings in your book.
Incidentally, a comment about fonts in general on any device, size them appropriately for the viewing distance. I don't have (and do need) reading glasses, so for handheld devices I need to make the font size quite small not large. This creates the effect of it being further away and thus easier for me to focus on at a close distance. If the letters are bigger, even tho they look nice, at a close distance my eyes cannot focus properly on larger letters anymore like they used to when I was younger. On a monitor it is the opposite, I need larger letters and I am further away from the screen. There are other technical things as well, but I wanted to mention the thing about size, because most people think they need bigger letters, but that's not actually the case every time.