This is the paper in which Knuth and Plass define the bones of the layout algorithm behind TeX. The Knuth-Plass algorithm decides how wide spaces should be on each line and which choices of hyphenation out of some predefined set should be used to lay out a paragraph. It's super readable and generally quite joyful. Knuth describes TeX as a "labor of love", and it shines through that paper.

(1981)