That is a histogram of his Amazon book ratings, and here is the source (HT TC). He is quite the pushover, and the above is a classic example of why a good rating system must rate the raters or suffer from bias. What about the content? Two trends stick out:
1) He is highly positive. Among his most common adjectives are "masterful," "remarkable," and "brilliant." Even when he gives a book four stars, he rarely says anything negative, and in fact it's usually not clear why books didn't get the top score of five.
2) He is highly technical. By this I mean that the majority of his sentences are devoted to strict summary rather than analysis. This makes sense, as it is probably smarter for a politician to say something obviously factual (and thus unassailable) than to take a risk.
