If I were writing a book on Traditionalism, I would consider each of the main figures in the movement -- Guénon, Schuon, and Coomaraswamy -- and show what their views were, how they came to hold them, and how they justified them. I would then conclude by looking at their legacy, both in the Traditionalist movement and in the wider intellectual world. So what has Dr Sedgwick done?
It is very difficult to get a clear idea of their philosophy from his book, because the whole thing is embedded in an excess of biographical detail. We learn why Guénon's PhD thesis was rejected, which schools, he taught at, and what his house in Cairo looked like; it all gets in the way.
The author also devotes too much space to peripheral figures, such as Evola and Dugin. Traditionalism is a largely religious movement, but Evola was basically a political thinker of the far right; he owed far more to Nietzsche than to Guénon. As for Dugin, the chief reason for including him seems to be that it gives the author a chance to give a highly-coloured description of a secret meeting in Moscow to get things off to a rousing start. As Sedgwick wrote to Fitzgerald "I'll be honest: one has to dress things up slightly.." Exactly.
If you want to learn about Traditionalism, read the originals. For only a couple of pounds more, you could get "The Essential Frithjof Schuon" or "Underlying Religion, by Lings and Minaar".