There is no way to change the contrast. If adjustment is needed, the typical approach is to change the text size. I like a bolder font, which is no problem for FBReader but not possible for the Kindle or for the iLiad with MobiPocket Reader.
I switched from a Nokia 770 to an iLiad (still using FBReader) as my primary reading device. If the line spacing was adjustable I might switch again to my Kindle, but perhaps not because an 8" screen really is better than 6" for reading. I still recommend the Nokia tablets, particularly if portability or casual internet access is a requirement. The other area where the color screen is a big plus is illustrated books (e.g. SciFi magazines). Also, the Nokia screen is almost exactly half the size of the 6" Kindle/Sony screen. In its usual landscape mode, it is the same width as the Kindle in portrait mode but half as high. So the reading experience is similar except there are twice as many page advances (which take no time at all on the LCD screen).