[SunkenCiv]

The idea that Oxford authored the plays (despite having died in 1604) didn’t originate in a book in the 1980s, it’s at least as old as 1920:

“Shakespeare” Identified
In Edward De Vere the Seventeenth
Earl of Oxford
by J. Thomas Looney
http://www.shakespearefellowship.org/etexts/si/00.htm

[KamperKen]

"The idea that Oxford authored the plays (despite having died in 1604) didn’t originate in a book in the 1980s, it’s at least as old as 1920

I believe Horace Walpole ("Castle of Otranto" author) in the late 18th century also theorized that Edward de Vere was the real author.