What you get out of Age of Darkness depends a lot on whether you have read all the other books in the series, and how much you remember from them.
Some people have criticised this because it does not cover the 10 year road to Terra that the Age of Darkness refers to in Warhammer history.
10 years in one paperback was never going to happen.
What we get instead is a collection of short stories that do, for those who know their Heresy, serve to throw some light on the events of that period - after all we can safely assume Games Workshop hasn't given up on further stories from this time period.
For instance, one story tells of how the remnants of one of the traitor legions remain loyal to Emperor and end up playing a vital role in the final part of the Heresy on Earth itself.
Another illuminates the doubts and fears of one of Horus' closest brothers.
The Sons of Horus betray their own, a desperate attempt to trap the Warmaster, Alpha Legion spies...... it goes on. Snippets of information that fill in the gaps of what is known, that mesh with aspects of previous books, to flesh out the history of the darkest time imaginable in the 40K universe.
The Last Remembrancer is perplexing, and will probably not be what the reader expects. In terms of plot points it delivers very little.
Oh, one last thing....Dark Angels. And hints that the role of the Ultramarines - that current 40K mythology places as being unable to reach Earth in time to effect the Siege of Terra or play much role in the Heresy beyond their battles with the Word Bearers - was far greater than known. Their Primarch has a plan....