TerryWalstrom, I too went to the Wikipedia and then compared the information there with typical indicators of books being sales successful. One of the indicators is the fact that the book has several editions (and apparently there was some kind of announcement made in 2015 of another new one, though Franz passed away in 2010) and his wife in 2013).
Also, I found that for books from 2001 to the current date, approximate sales data may have been gathered by a company named Nielsen Book Scan, a subscription service marketed mostly to publishers and bookstore chains. However, actual numbers are considered proprietary information.