Check out the review of Michael Kaminski's Secret History Of Star Wars:
The Secret History of Star Wars Review - Comics Review at IGN
This has been available online for quite some time so I don't really know why anyone would want to buy it (especially at the current retail price, which is just ridiculous).
The review on IGN is spot-on, though, and reflects my own impressions when I attempted a read-through a couple of years ago. The book offers no new interviews or insights from the people involved with the creative process and instead comes across as a sprawling attempt to discredit Lucas at every turn. Indeed, if Mr Kaminski had simply called his book Why I Hate George Lucas it would have been a more accurate summation.
As I say, I only attempted to read it when I found a link online. I found it very hard to get through, partly because of the tone but also because of the style of writing. I read enough of it, though, to come to the conclusion that Michael Kaminski seems the sort of person who would turn up at the doors of Skywalker Ranch to accuse Lucas of stealing all his good ideas so he could pass them off as his own. A bit like Stephen King's Secret Window, Secret Garden really.