This is not a weekend read. Andrew could probably take some of the highlights and cut this 1000-page behemoth into something much more digestible, but if we he did, it wouldn't be a history any more. Objective, fact-based (with an endnote for nearly every sentence to prove it), and detailed, Defend the Realm really packs it in. The density of information is high, the amount of filler is essentially nil, and the type is quite small.
Andrew does not speculate as a writer of a general audience work might. He's a professional historian and this is a professional piece of historical scholarship. I've been reading this book on and off for six months, and some casual readers might have given up before then. It's not narrative and we know only what definitely happened; he does not narrate the emotional states of the principal players or speculate on the dramatic tension at some of the events.
On the other hand, the material sometimes speaks for itself. This is the real-life story of war, foreign spies, secret political meetings, terrorists, and narrowly-evaded disasters of all kinds. For hard-core history/military/intelligence buffs, this is a goldmine of carefully collected and organized material. The shadowy realm of military intelligence is a rarely thought about but inescapable part of our modern world.