The J Dictionary is the authoritative and definitive specification of the J language. It can be used to learn J, but the fact that it covers all of the language concisely, yet completely and rigorously, with more emphasis on the complex than the mundane, does scare some of us away.
This online book provides a kinder, gentler start for beginners. This book takes you along a path in easy steps to the point where you can write an application in J. Along the way you will be introduced to all the key ideas in J by seeing them in simplified and specific contexts. At the end, you will be able to write real programs in J, and you will also be comfortable in using the J Dictionary as a reference for your work as a J programmer. The purpose of this online book is to get you up to speed where you can use the J Dictionary in a manner that makes you wonder why you ever bothered with this simple stuff.
You should be able to work your way through this book fairly quickly, and at the end you will be an entry-level J programmer. As such, you will have far more programming power at your fingertips than even the most experienced Basic or Java programmer.