1. I have changed the category to Processing and added the tags gmic, rawtherapee, scanner. I think we have room to discuss how to make better scans.
2. iarga is correct. Clean Text is available as a GIMP-G’MIC plugin filter and also as a CLI command (afre_cleantext). Unfortunately, I am having trouble retrieving it from the servers. I don’t know if you have the same issue ATM when you try updating the filters. Do the following:
For GUI plugin
Then search for Clean Text.
For CLI
gmic update
gmic sample tiger afre_cleantext
3. You can totally do batch work using the GUI or CLI. Try the filter on the two most different scans and find a happy medium.
Then in GIMP, Open as layers..., in the plugin, set Input layers as All, and then you could apply the filter to all of the pages.
Or in CLI, you could copy all of the scans to a single new folder and do
gmic input_glob *.jpg repeat $! local[$>] afre_cleantext , o _{b}.jpg endlocal done
Hint: replace the comma with your parameters. The comma by itself means that it will use the default values I have set.
PS If you found my filter and instructions helpful, perhaps we could as a community write a tutorial together, with instructions on how to digitize a book. I don’t have the time for that currently but what do you think @patdavid @paperdigits?