I would like to know the top-twenty (or thereabouts) bestsellers in
non-fiction books (Sachbuch) for the last few decades taken together.
This list should contain some rough estimate of sales (in money or
numbers) and apply either worldwide or to the US or to the UK or to
Germany (will pay extra if I get separate lists for more than one),
and refer to a period considerably longer than just the last few years
(anything in the region of the last 20-50 years or even referring to
'all times' would be fine). The data may have been compiled at any
time during the past decade.
Please note: I will not pay for the German FOCUS estimate of such
bestsellers published on 10 April 1995 since I already have this.

Hello Cornelius2,

The web site of Cader Books provides Publisher's Weekly's list of top
ten bestselling hardcover books for each year of the 20th century up
to 1998, with separate fiction and non-fiction lists (non-fiction
lists start in the 1910s section).  However, only titles are given; no
sales data is supplied.
http://www.caderbooks.com/bestintro.html

Alcholics Anonymous claims its Big Book is an all time bestseller
"Since publication of the first edition in 1939, close to 22 million
copies have been sold. It took 35 years to sell the first million
copies of the Big Book. Now A.A. distributes more than a million
copies each year in English alone. The nearly 600-page volume has been
published in 43 other languages, including Arabic, Croatian, Hindi,
Mongolian, and Punjabi. Translation into Simplified Chinese, used
throughout the People’s Republic of China, is currently underway."
http://www.alcoholics-anonymous.org/default/en_release.cfm?newsid=6