I bought it at a Half Price Book Store and gave it a read (I probably own at least 50 books on songwriting, theory, etc.) - I practically live at Half Price Books and buy everything I find on those subjects)
I thought it was OK at a fairly basic level - and could be used by someone with no real songwriting knowledge. It does provide some interesting historic data on various musical genres and examples of songs as a point of reference.
It did little for me, however given that I've been writing for about 30 years and have read all the Shiela Davis books, and dozens more I suspect I would have been surprised if a "Dummies book" would have provided some profound new discoveries.
That being said, it was well written and provided some entertainment. It is a light read that doesn't get too technical (some very basic theory), designed for a novice (as most "for Dummies" books are designed to do).