K2pdfopt now has gamma and contrast adjust options that might help the PDF show better (darker) on your Nook. I don't have a Nook so I haven't tested it on that device, but you can try setting gamma to a lower value using the "g" option from the interactive menu. It already defaults to a lower value than it used to (0.5 instead of 1.0, starting with k2pdfopt v1.19, released on 9-2-11). You can also use the "o" option to set the output DPI to a larger value which will magnify the text more.