You should visit the treasure trove over at the DIY Bookscanner forums:
http://www.diybookscanner.org/forum/
There is a ton of tutorials/information over there on what people do to fix up scans. Everyone will have slightly different workflows... and there are probably some very in-depth Imagemagick commands that people have come up with to do what you want.
ScanTailor is probably the easiest way to do a lot of this, but it does "recreate" the images/fonts/text, instead of just taking your image and doing basic cropping/centering.