image processing to improve tesseract OCR accuracy

1. fix DPI (if needed) 300 DPI is minimum
2. fix text size (e.g. 12 pt should be ok)
3. try to fix text lines (deskew and dewarp text)
4. try to fix illumination of image (e.g. no dark part of image)
5. binarize and de-noise image

There is no universal command line that would fit to all cases (sometimes you need to blur and sharpen image). But you can give a try to TEXTCLEANER from Fred’s ImageMagick Scripts.

If you are not fan of command line, maybe you can try to use opensource scantailor.sourceforge.net or commercial bookrestorer.