Clarissa
This repo contains the full text of "Clarissa; or, The History of a Young Lady: Comprehending the Most Important Concerns of Private Life. And Particularly Shewing, the Distresses that May Attend the Misconduct Both of Parents and Children, In Relation to Marriage", by Samuel Richardson, published in 1748, which is one of the longest novels in the English language, and which is also conveniently in the public domain.
It is a useful data set for testing the ability of text editors to deal with extremely long files.
wc -w clarissa.txt reports 957171 words.