Links of Relevance:

JSoftware: the company that produces J

This wiki is a central point linking to introductory texts, documentation, examples, tutorials, on-line books, articles, essays, etc.
Easy-J: a small introductory book
J primer: the ‘canonical’ introductory text from JSoftware
Learning J: a tutorial book covering all aspects of the lanhuage
A brief J reference: may also be used as a detailed language overview
J dictionary: the language definition
J vocabulary: a part of the J dictionary – a list of all language items linked to their definitions
J phrases: a collection of useful definitions; this is both the ‘standard library’ of J and a resource to learn from
Other books and a reference card
Articles (those in the At play with J series are of pronouncedly educative nature)
Essays: commented examples of J code
Puzzles: problems challenging both as J programming problems and otherwise

Vector, the journal of the British APL Association publishes articles on J and maintaines on-line archives

A personal view of APL, by K.E. Iverson: discusses the evolution of concepts that lead from APL to J

Computers and mathematical notation, by K.E. Iverson: discusses J as the blending of mathematical and programming notations that improves on the former in substantial ways

Functional programming and the J programming language: an article

The J Incunabulum: the very initial J interpreter – in one page
Personal web pages with J resources
Oleg Kobchenko: software, links, other information
      J by example is a good way to get acquainted with the basics of J in just several pages
Keith Smillie: programs, articles
Donald McIntyre: articles on historical and technical topics (some technical details are outdated re. the current definition of J)
      The role of composition in computer programming: many examples of the various forms of composition in J