The first article I read about the techniques used in the spell program was the 1985 May issue of Communications of the ACM (CACM for those who know), https://dl.acm.org/toc/cacm/1985/28/5, in Jon Bentley's Programming Pearls column.
Not as much detail as the blog.codingconfessions.com article mentioned above, maybe some of the other/later techniques were added later on?