The Contents (help) file that comes with 3dgs when you buy it, I feel does a good job of explaining most of the features and scripting. Anything else can be found or asked on the forum.
Why a book?
Can you ask a book for help in a specfic script help?
Is the book up to date with the latest "A" verson?
Has the author actually published a commercal work using 3dgs?
Has he written books before?
Is the book comprised of orginal ideas and not an compilation of resources?