Reason number 1 - This book is for JBossAS-5.0 version of the server. Right now, there are no other books available for JBossAS5 and this is the only one
Yes, there is quite a bit of free information about JBoss technologies on the internet. Some are decent, others are not worth the bandwidth necessary to transfer the bits to your browser. What we have done is distilled the information into a format that is easily readable. In addition, our book is focused on the practical side of things - we show you how to get things done.