[AnotherCat]

I think that JSWolf's comments regarding E Ink working differently than LCD are absolutely correct (all else being equal). Further to what he said is that an important part of grayscale display quality is the contrast handling ability of the display. I have not followed E Ink for a year or so on that but as of not very long ago E Ink was capable of only little more than paper, whereas LCD is orders of magnitude better.

Because of this one can expect E Ink to be poorer in displaying greyscale images than LCD, other things being equal. I read a lot of material having grey scale images (and color too) and so no longer use an E Ink reader.

Your Figure 3.1 is a color image (if I am looking at the correct item, I have not followed the whole thread) and if comparing readability of the small grey font as between the Voyage and the Nook then much more than just resolution also comes into play as to readability. For example, contrast through the choice of colors (which is in the hands of the author) and the accuracy of rendition of those by the display and the display's background may mean that on an LCD display with high resolution an image may appear poorer in readability than on a lesser resolution, lower contrast display in greyscale.

In case not known, on color display Android devices one can force them into "Monochrome" by accessing Developer Options (if not open already then Settings>About>then Tap "Build Number" until the options open and are placed in the Settings menu). It varies between versions and devices but under something like "Simulate Colour Space" one can select "Monochromacy". I think the Nook is Android 4 in which case, although I recall the option is still there, it may have a different name.

Viewing the image in "monochromacy" (I don't know how many shades of grey it will be but on recent devices it results in quite good greyscale) on the Nook, if possible, may give a better comparison with respect to the comparable resolution of the 2 devices.

In the end it is wrong to compare the "resolution" of a color image on one device with the same image rendered into greyscale on another, all else being equal, as it is the "readability" that one is comparing. My pick is that one can always produce a far more readable image on an color LCD device compared to an E Ink one, all else being equal, and if one wants to maximize the readability on both then one should maybe start with the color one and select colors and contrast (including the background) that when rendered into greyscale maximizes greyscale readability.