For me, the most exciting part of the late 2011 iBooks 1.5 update was seeing Apple look beyond the old system typefaces (designed for another time and medium) to contemporary designs, including those from smaller, independent foundries. Yves Peters described these new font additions on the FontFeed. See that article for comprehensive screenshots in Normal, Sepia, and Night themes.
The image above was captured from an iPad 2. The images below are from a (Retina) iPad 3. Of course, all are scaled to fit this site.