Thus, by definition, "scripture" would not be "scripture" without something outside of scripture (Christians themselves) to determine what scripture is.
Ultimately, I don't believe in "sola scriptura" because I believe that God can (and does) teach us to follow Him in so many different ways, different ways that do not always involve scripture. That said, I do believe that scripture is ultimately the core and basis of our Christian faith. In addition, it is immensely beneficial in teaching us to seek God's wisdom in everything we do.
Back to the book: I disagree with the author's assertion that scripture alone should be rejected because the Church itself confers essential parts of Christianity. So what's wrong with this? Nothing, if we consider the body of believers to be "the church". Unfortunately, the author very strictly interprets "the church" as "the Roman Catholic Church", and rather than simply acknowledging that Christians can learn from each other, he instead refers to Christians having an "infallible" leader on Earth (referring to the Pope). A simple survey of the history of the Catholic Church reveals very obviously the lack of "infallibility" of the Pope. Ironically, the author rejects the idea of sola scriptura on the basis of evidence that points to its logical rejection, while embracing wholeheartedly that the Catholic Church is the "one true Church" and that the Pope is "infallible", despite the overwhelming evidence to the contrary.
Overall, a thought-provoking read.