Well, since you're only interested in scanned images, most any image-editing tool should work. The ImageMagick toolkit has a bunch of command-line tools which are easily scriptable, ie with PERL or Bash. The "convert" tool is a swiss-army knife for cropping, etc. I'm not sure about skew correction, though.
Also, a quick Google turned up this: http://www.gnurou.org/blog/2007/01/0...e_ebooks_linux