* Use 'StateT'.
* Add 'replacements' and 'corrections'.
* Add 'toLower' handling.
* Add 'flip' to 'apply'. Some renaming.
* Use some better (function-) names.
* Introduce 'BookDataMap'.
* Add 'usage' and some documentation.
* Add replacements and corrections.
* Add 'corrections' and some documentation.
* Add handling of environment variable 'HOME'.
* Add function 'readDirPath'.