Articles/APL - a Glimpse of Heaven
The Vector article "APL - a Glimpse of Heaven" (link below) is an excellent read that highlights the benefits of using APL. However, while it was written specifically for APL, it applies equally well to J, and might just as well have been "J - a Glimpse of Heaven".
The J reader needs minor changes to convert the APL into J - this would be an interesting exercise for the beginner. Also, you have to allow for the author's enthusiasm for the non-standard APL character set.