I love it too and the font is called Georgia.
I hated that on iOS 7, and subsequently iOS 8, they changed to that San Francisco font — that was even one of the reason I didn't update my iPads.
On my newest iPad I couldn't go back and since I was struggling with iOS 8.3 I decided to try the iOS 9 Beta released last week and gues what? Now you can personalize more stuff on Reader Mode. Like before, you can change the size of the font, but now you can also change the font type (and Gergia's included) and also change the background, there's a Night Mode as well. Real nice.
What's not so nice is that if you share a web page with Reader Mode on, the content will no longer be emailed, only the link.
I hope it's a bug, after all, it's only a Beta version, but...