Ostensibly, this book is about a simple question: are humans alone in the universe? It has to go the long way around to come to any answers, exploring other arguments by way of figuring out whether the Earth is or isn’t rare in the universe and whether or not life is as tightly constrained as some people say, but the core principle of the book is that we need to find a middle ground between the current main ideas — the Copernican view that we can’t be unique, and the Rare Earth view that says life in the universe must be unusual.
Mostly, my wife got to watch me mutter “yes, obviously”, and I’m tempted to quote Lord Peter on Chief Inspector Parker here — it takes Scharf a desperately long time to someone who already has a somewhat formed opinion to “crawl distantly within sight of a conclusion”. That conclusion, in the end, is basically where I stand: not enough data, come back later (with a side of Scharf being pretty sure that neither extreme is going to turn out to be correct, with which I disagree — I think it’s all up for grabs at this point).
So anyway, if you want to know why I came to the conclusion I’ve written in my science blog recently (i.e. “we don’t know and we can’t know based on the current data we have”), this book has a good roundup of the evidence. Scharf isn’t bad at explaining it.
But if you’re looking for answers, I find it as unconvincing as all the other attempts at answering this question.