Check out http://www.infomedia.net/scan/

Good general information on scanning, selecting resolution, etc.

Some of the hardware information is slightly out of date but the general info is very good.  Clearly explains the relationship between the original image (document, photo, etc.) size and the required scanning resolution to achieve the desired published image.

With a little web crawling you can find many other sources of good information.  see also:
http://www.ahip.getty.edu/intro_imaging/0-Cover.html

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