I prefer the natural look too, particularly if the book being scanned is "naturally" nice looking, with light pages and rich colors. That's the ideal; like a book fresh off the press. For then the scanner can leave the settings on "default" and get nice results. If only life were so simple.
When I first began scanning--not that long ago--I tended to think much like our friend Octal, here. That is, the less done to the scan, the better. The "Give it to me, warts and all" approach. I think I felt that way because I had come across so many over-corrected or "hyper-corrected" scans, and frankly, I was pretty discouraged and a little angry. Compared to all that hyper-corrected material, the natural look seemed so refreshing, and I still feel that way. Comics age "gracefully", and the natural softening of colors and yellowing of paper is much to be prefered to garish colors, and bright pages that make me what to squint. Indeed, I will not read such material. I now have a backlog of truly nice scans to read, so why waste time on dreck?
Now that I've been around the scanning community for awhile, I have seen what wonderful work some scanners have been doing. The best scanners, I now feel, are doing moderate correction to lighten pages, but doing so without color shift. That's the key; getting pages lighter without shifting, or overshifting the color balance. Hyper-correction is typically accompanied by skin tones that have shifted toward the blue, resulting in flesh tones that are electric-pink or hot-pink. Nothing is more disconcerting to me as a comics reader, than a comic with hideous pink flesh tones. Such results seem much worse to me than leaving the page entirely natural, dark, or yellow.
Doing page lightening properly, I think, is an art. It takes a good eye, and also a sense of what the original page looked like, back in the day, when it was fresh off the press. I find it all too easy to over lighten, and I would caution newbee scanners to approach lightening cautiously, especially at first. There is more to it than simply adjusting gamma. Only after some experience and a good understanding of color theory, as well as a feel for the lightening process itself, should more agressive lightening be attempted. In any case, I never try to make the page look whiter than fresh newsprint, which is actually off-white, even when fresh. And I always try to maintain natural unaltered flesh tones. If the flesh color looks off, I know the page is hyper-corrected or the color unbalanced.
As for contrast, again, moderation is important. Most people seem to prefer a little "pop" in the correction. By this, I understand them to mean a contrast bump (10-30%). The trouble with this is that every monitor is different. What may be a 10% contrast boost on your monitor, may end up looking like the page has a 30 or 40% boost on another monitor. Still, there are those books that suffer from weak or faded colors, that will benefit from a bump in contrast. How much to lighten pages, and knowing when to apply a bit more contrast, are both part of what I think of as the "trained eye" aspect of comics scan correction. "Experience is a great teacher", and "all good things in moderation." Two aphorisms to live by.
Addendum:
One more thought occurs to me now, a day later. Just another pointer for the beginning scanner. Aim for consistency. Experiment on two or three different pages within a book, to find optimal correction settings, then use those same settings throughout the entire book. Only make slight changes for an odd page that may be suffering from a special problem, such as overly light or dark ink compared to the rest of the book, or a splash panel that oxidized faster than the interior of the book. Even then, altering settings should be resisted. It's better to let the reader see such deficiencies, than to send him running repeatedly, to his viewer's correction settings.
