Books
APL in Practice: What You Need to Know, to Install, APL Is Easy, APL Programming and Computer Techniques, APL With a Mathematical Accent, APL, an Interactive Approach, APL/360: An Interactive Approach, APL2 at a Glance, APL2 in Depth, APL: A Short Course, APL: The Language and Its Usage
Conferences
APL 1996, APL 1997, APL 2000, APL 2001, APL 2002 (i), APL 2002 (ii), APL 2003: Call For Papers, APL 2003: FCRC conference, APL Conference
Consultants
Adaytum Home Page, APL Borealis, APL Consultants and Software, Causeway Graphical Systems Ltd., CoherentSystems: CoSy, Customized Applications, Inc., Dinosoft Oy, Lescasse Consulting, Optima Systems, Resources & Results
Interest
About APL (A Programming Language), APL and J, APL FAQ, APL White Pages, APL-ASCII Transliteration, Benchmarks of APL vs. Other Languages, CAFCA, Catalog of compilers: APL, Function Definitions in APL, Good Code/Bad Code & Looping
Personal Pages
APL White Pages, Jim Weigang's APL Information, Oleg's J Page, Présentations AFAPL, Rex Swain's Home Page, Sam Sirlin's APL Page, Timothy A. Budd home page, Yoshi's APL Page
Resellers
APL Borealis, ASTRAC Ltd, Simcorp, The Rochester Group, Inc., Vector's APL Product Guide
User Groups
ACM SIGAPL Home Page, APL Organizations, Association francophone pour la promotion du langa, Capital PC User Group (CPCUG) Home Page, FinnAPL Home Page, The British APL Association, Vector's APL Product Guide
Vendors
A+, APL2000 Inc., APL2C, CoSy/K/CoSy, Dyalog Limited, IBM APL2 : Overview, J Software, Juggle: J for Unix, Kx Systems, MicroAPL Ltd
Brighton University (UK)
The APL section of "The Brighton University Resource Kit for Students". The higher levels of this site are well worth studying as well.
TechEncyclopedia
Many computing related definitions here
Why APL?
Anonymous essay from 1983 published by ACM SIGAPL, which explains the strengths of the APL approach to algorithm design.