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.
Frank Liang's thesis on hyphenation (developed for/used by TeX), supervised by Donald E. Knuth:
Liang, Franklin Mark (1983) WORD HY-PHEN-A-TION BY COM-PUT-ER, Ph.D. thesis, Stanford University. https://www.tug.org/docs/liang/liang-thesis.pdf