I don't see much added value for this book. FreeNAS, like most opensource projects, is poorly documented. That is, the user forum on the web is the documentation. So writing a book about a good opensource project is a very good idea.
FreeNAS has a very user friendly web interface, with a lot of text explaining the meaning and effect of each tick box or input field you can find in it. This book is basically the synthesis of all the text in the webGUI. So installing FreeNAS on a left-over PC and browsing through the webGUI will give you about as much information as contained in this book.