This is a short biography of several intellectuals (Rousseau, Shelley, Marx, Ibsen, Tolstoy, Hemingway, Brecht, Russell, Satre, Gollancz, Hellman, Wilde, Connolly) and a few others (Wilson, Waugh) , specifically seeing if their lives qualified them to say what they said. For example, did Marx know what the conditions of the working man were like? Did Rousseau treat his children well?
This is a good book. Johnson shows, time and again, how some of the people whose ideas most affected the modern age were basing their thinking largely on their own egos and a "creative interpretation" of the evidence.
He defines "intellectual" as someone who effectively rejects the whole of human knowledge to that point and assumes that they can do better. So it is not surprising that the people he surveys are egocentric, deeply troubled, and do not live up to what they preach.
A good quote from the book:
It is a fact, and in some ways a melancholy fact, that massive works of the intellect do not spring from the abstract workings of the brain and the imagination; they are deeply rooted in the personality.
It certainly highlights all the more clearly how distinctive Jesus and his followers were in the history of people with radical ideas, and how little basis there is for accepting so much of the modern worldview.
However, I can't help feeling that Johnson himself falls into some of the traps he highlights in his selection of intellectuals. By having all people who hold fairly similar views, he strengthens the case for anything opposing it, specifically his own.