Paul W. Abrahams, Kathryn A. Hargreaves, and Karl Berry. TeX for the Impatient.
Victor Eijkhout. TeX by Topic.
Donald E. Knuth. The TeXbook.
Norman Walsh. Making TeX Work.
Karl Berry and Oleg Katsitadze. Eplain 3: Expanded plain TeX. TUGboat 26:3, 2005.
Oleg Katsitadze. Implementation of hyperlink support in Eplain. (Describes the state of code in May 2006.) Document sources are also available here.
David Walden. Travels in TeX Land: Word2TeX redux, TeX2Word, plain TeX with Eplain, and playing with “thought breaks”. The PracTeX Journal, 2005(4).
Comprehensive TeX Archive Network.
TeX Frequently Asked Questions.
The TeX Users Group (TUG).