If you watch Doctor Who Confidential, or listen to the podcasts they post on the internet (or the commentaries that are on the DVD box sets), then you will probably already have a pretty good insight into why things have been scripted as they have been in Doctor Who (at least from the "artistic" point of view!).
What this book tells you is how that process happens.
To add to what Michelle has said, the book doesn't just include snippets of the scripts, it contains the full first drafts of Voyage of the Damned, Partner's in Crime, The Stolen Earth and Journey's End, as RTD wrote them - hot off the keyboard!
I found the book to be very interesting; I'm interested in Doctor Who anyway, but I'm also interested in story development and writing generally, and this was a very good marriage of both.
I would say that if you are an aspiring script writer - regardless of whether you like Doctor Who or not - this will make an interesting read. If you are interested in Doctor Who as well, this is a must-read.
And, for those who don't yet have a copy but want one, check out The Book People's web site, as they were selling it for just