[jaybee]

Go to the settings page. That's the icon with gears on it. Tap "general" then "accessibility." You can select "white on black" to invert all colors on the screen, including, of course, the pages in any reader. This is a little cumbersome to do each time you read, but there is a shortcut. Still on the "accessibility" page, go to "triple click home." You can set this to toggle white on black. Then just triple-click the home button to reverse the colors or to cancel and go back to standard colors.

By the way, I'm very happy with iBooks.

(Sorry, I just read the title of this thread and see it refers to the iPhone. My comments refer to the iPad. I'm leaving them in case they may be of use to anyone--the process is a little obscure, but is very useful to me.)