SQL Server White Papers vs Books Online - What's the difference and where are the white papers?
How are white papers different from books online? Are white papers
  just a deeper dive into a technical issue?


Books Online is the documentation, while white papers are often more technical discussions of a topic, some times with an lean toward marketing.


  Does Microsoft have an organized place for white papers? If so, where
  please?


Microsoft has an gallery of SQL white papers up through SQL Server 2014 here: https://social.technet.microsoft.com/wiki/contents/articles/13146.white-paper-gallery-for-sql-server.aspx

White papers for SQL 2017 and 2019 are here: https://info.microsoft.com/sql-server-2017-technical-white-paper-registration.html and https://info.microsoft.com/ww-landing-SQLDB-Microsoft-SQL-Server-WhitePaper.html

An SQL 2019 white paper focusing on big data is here: https://info.microsoft.com/ww-landing-SQL-Server-2019-Big-Data-WhitePaper.html

More can be found here: https://www.microsoft.com/en-us/search?q=white+paper+sql