One thing lacking from the updated iBooks app that’s now available for iPhones is the ability to read with white-on-black text. Black-on-white and black-on-sepia are your only native options.
A hackish way for getting white-on-black (my preference for reading in a dark room) is to go into the iPhone’s General Settings -> Accessibility and setting the “White on Black” switch to “ON”. Now iBooks is in white-on-black, but so is everything else on your iPhone, and to make everything “normal” again when you’re done in iBooks, you have to go back into your general settings to turn white-on-black off. Kind of a pain, but it works, mostly.
Come on, Apple, make white-on-black a standard color choice in iBooks. (You know, like the Kindle app does. Just sayin’.)
(P.S. For those who might be wondering, some people with vision impairment are sensitive to light, which is why most accessibility settings in computers offer a white-on-black or inverted setting.)