plain TeX

• The TeXbook
  the definitive guide to TeX by the system’s creator by Donald Knuth
• TeX by Topic
  systematic reference manual by Victor Eijkhout
• A Gentle Introduction to TeX
  a manual for self-study by Michael Doob
• TeX for the Impatient
  free edition of the book by Abrahams, Hargreaves, Berry
• Getting started with Plain TeX
  by D. R. Wilkins
• Making TeX Work
  republished GNU FDL edition of the book by Norman Walsh
• TeX Primitive Control Sequences
  HTML version of TeX Reference Manual by David Bausum
• Das kleine TeX-Buch
  german translation of Donald Knuth’s TeXbook by Fritz Cremer
• Einführung in TeX
  pdf version of the introductional book by Norbert Schwarz