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?

Link to the online version of the 1985 May Programming Pearls column: https://dl.acm.org/doi/10.1145/3532.315102

The PDF version of that article: https://dl.acm.org/doi/pdf/10.1145/3532.315102