Unless you've already returned your PRS-900, you could try and change the font on it. I found the default font a bit hard to read in some lighting conditions, and changed the font to the bold version of another font (whose name slips my mind at the moment). You can find directions on how to change the font in
this thread.
By making the font a bit bolder I found that it was much easier to read on it and no longer have any problems with it. The PRS-900 is the only e-reader I've ever used though, so I can't say how it compares with other e-readers.