iOSFonts.com is a handy reference guide to all the fonts built into iOS devices. This is handy if you plan on referencing any of these fonts in your ePUB projects.
If you view the site in Safari on an iOS device, you will see the actual fonts. If you view the site on a desktop browser, most of the fonts will simply display in a browser default font. The site allows you to see which version of the operating system supports each font.
Sadly, Apple hasn't seen the light and added Hobo to the list of fonts built into iOS. Oh well, one can dream!