Well, first off, Geoffrey Williams is in fact the name of a UK singer/songwriter who wrote several pop songs that made the charts in the last 30 years. So at least he has some experience and isn't like these people on ExpertVillage or something.
The next thing I look at is the testimonials. He's got two or three testimonials from actual performers who give links to their pages, one would assume with music on them. This is perhaps the best acid test of all. Go listen to those people's songs. If they all sound like utterly dreadful crap, then it's obvious that Mr. Williams is just trying to throw a spanner in potential competitors' works.
Then do some brutal self-evaluation of you - your background, your skills, your ambitions, and your ideas. Do you have the general understanding of music to write a catchy hook and something compelling-sounding, on piano, keys, or guitar? One can compose on other instruments, of course, but I would recommend one of these. How about your lyrical skill? Where is that at? My point about this is, the book probably won't make up for any weaknesses you might have in these areas.
Where books like these do shine is to give you a better understanding about why songwriters do certain things they do, examined against the realities of the music business and the listening public. You probably know what you like about the music you like, if you think about it, and likewise you can tell someone what you don't like about a song you hate. That should shape your songwriting.
In the end, I think this book might give you a lot of insight and ideas, but there are a lot of ways towards writing songs, and I would be very surprised if you couldn't find as much inspiration, advice, or ideas in other ways (internet, songwriting workshops, college courses, interviews, etc) without paying the cost of this book package.
"Jus suum cuique"