Where might I find a list of the best selling books (fiction and
non-fiction) for the decade 1980 - 89?

Here are several lists of 1980s bestsellers, including both fiction and nonfiction:

University of Illinois at Urbana-Champaign: Annual Bestsellers, 1980-1989
http://www3.isrl.uiuc.edu/~unsworth/courses/bestsellers/best80.cgi

Hyde Park Books: Best Selling Books in America, 1980's
http://paperbarn.www1.50megs.com/BestSellers/1980s.htm

The Eighties Club: Bestselling Books of the 1980s
http://eightiesclub.tripod.com/id359.htm

And here is a list of the decade's top 25 fiction bestsellers:

University of Iowa: Top 25 Fiction Bestsellers of the 1980's
http://www.uiowa.edu/~c07s193a/Book_Lists/1980sBestSellers.htm

Google search strategy:

Google Web Search: "bestsellers" + "1980" + "1989"
://www.google.com/search?hl=en&ie=UTF-8&q=bestsellers+1980+1989

Google Web Search: "bestsellers" + "fiction" + "nonfiction" + "1980s OR 80s"
://www.google.com/search?hl=en&ie=UTF-8&q=bestsellers+fiction+nonfiction+1980s+OR+80s

I hope this is precisely what you need. If it is not, please request
clarification; I'll gladly offer further assistance before you rate my
answer.

Best regards,
pinkfreud