Basic concept of TEI (and of text-encoding in general) as a markup approach to digitizing literary and other texts:

“The TEI was founded in 1987 to develop guidelines for encding machine-readable texts of interest in the humanities and social sciences. Its work was supported by the Association for Computing and the Humanities, the Association for Literary and Linguistic Computing, and the Association for Computational Linguistics, and received generous grant funding from the Mellon Foundation, the EEC, the National Endowment for the Humanities, and other institutions. The “P3” Guidelines were delivered in 1994, and have become the de facto standard for encoding of literary and linguistics texts, corpora, and the like.” (from the TEI FAQ)

Starter Links: TEI home | See also Michael Sperberg-McQueen’s “A Gentle Introduction to SGML” for an overview of the text-encoding or text markup concept