Hi Jim,
This is a tricky subject. Some scanners out there would rather you not adjust the scans as they feel a book should be as close to 'as-is' as possible. To share them as an museum artefact if you will. While some others would say feel free to use the scans as you may. I always suggest contacting the scanner if know here in a Private Message (the word balloon icon beside the envelope icon below their name) and see what they say.
If you see a book that says 'raw' in the title those are posted with the hopes someone might someday edit them so feel free to work on any of those.
Finally, when sharing your finished results please share them in the same section as the original and add (AJ edit) or (re-edit) to the file name. And always give the original scanner full credit in the description section. Adding what you did to the edit is always interesting to know as well.
Good luck with your project,
-Yoc
btw, if you hit Preview in your message you can show jpgs (please, keep them in the 800 pixel height range) by clicking on 'Additional options'.