I believe that the text files are automatic conversions from the djvu files, and the file extension is preserved, even though the result is txt.
I convert the text files in exactly the same way as I convert any other text file. I drop the file into a Doc and then edit, befor using either Book Designer or Calibre for the final conversion.
Unfortunately, the Internet Archive text files are of very poor quality, and require many hours of proofreading before conversion.
1. You will need to strip out the headers and footers.
2. You need to restore the italics.
3. You need to correct the OCR errors.