Books For Learning J
A 44-page introduction to J for newcomers to the language. The authors provide this without copyright and it is available for free distribution and it can be edited or altered without acknowledgement.
The J Primer is a somewhat linear approach to learning J written one of the developers of the Interface. As its title suggests, it attempts to be an introduction, using mostly the explicit definitions of verbs, a simple program to convert between Fahrenheit and Centigrade, some file usage, some plotting, and even some GUI development.
The text of this book has been added to the releases of J and is part of the online help from JSoftware. 
Eric B. Iverson -Copyright Jsoftware 1996 - 2011
An excellent introduction and overview of the core J language.
Roger Stokes - Copyright Roger Stokes Available in a J installation or here.
J for C Programmers
This book is a guide for making the transition from scalar languages to J. Early chapters describe J in terms of C, but the reader is weaned off C as the book progresses, and many J programmers ignorant of C have found the book a useful companion.
Henry Rich - Copyright Henry Rich
The book has been translated to German, see File:J fuer c.pdf.
J Reference Card
The J Reference Card, a 2-page summary of J primitives, is available File:J602 RefCard color letter current.pdf
A German transcription on 6 pages is available here File:J schnell referenz 20070616.pdf.