Book number 41 in The Horus Heresy series, The Master of Mankind attracted me because the character of the Emperor is always being talked about but never in detail in any Warhammer 40,000 lore. We can learn how he did this and that, how he fought, how others think he decided a certain strategy... but never get to actually see him in "direct action", engaged in a conversation with someone, or simply get to the time where he was not in the Golden Throne and how he lived and directed mankind then.
The book focuses indeed on the Emperor, both in how he was directing some of his main projects back in the Horus Heresy (the Eldar Webway, the Golden Throne, and other which I'd rather not spoil) and how he thinked, bits of his personality and extremely radical practicality regarding every action he took, in the form of half-dream half-visions he shares with some of his high-ranking Adeptus Custodes officers.
But we also get to learn qute a bit about the Adeptus Custodes themselves and the Sisters of Silence, as the problems that Magnus the Red caused spawned terrible chaos creaures pouring in into the webway, and mankind needs to keep hold of it at any cost.
An interesting book, as I mentioned with hints at the Emperor's personality, good battles full of tension, chaos demons, titans... The perfect mix for an entertaining read.