From 31-40 of the Horus Heresy there are 5 collections of novellas and short stories, mostly from earlier in the Heresy.  The Silent War tries to impose a guiding theme, but it’s pretty imaginary.

The Silent Phoney War

As I understand The Silent War, we get insight into how the Imperium defended itself at the highest level.  While preparing the defences, the regent Malcador sends special forces out on critical missions such as:

 ...obtaining the Rosetta Stone.  Apparently it’s important for hope. 

Viewing the collection holistically, the whole conceit is a joke. The reality is that Malcador serves as (weak) exposition.  He’s the titular Charlie sending his Angels flouncing off on their self-contained adventures.

The main point is that the galactic conflict remains hidden from us.  There are no grand strategic insights because there is a pre=set narrative to the Horus Heresy and the authors don’t really have the room to weave it into a struggle across a million worlds.  The Horus Heresy is a small universe, which is fine, but it would be laughable to pretend this collection changes that impression.

Otherwise the stories are mediocre, with a few good ones, but nothing outstanding.  The “fix” to The Outcast Dead requires head-splitting mental gymnastics:
 
Nagasena tries to process this information, but the pain from his wounds is overwhelming his thought processes.

 The Purge

As for the main novella, I guess I will begrudgingly transfer over my review (and rating) for The Purge across to this collection.  The novella does not really fit the collection, but Games Workshop had to stuff it somewhere, I guess.

The champion had never been beaten blade-to-blade, but as he turned to find a new foe, a stray bolt from out of the smoke took him in the eye. It punched through his left visor lens and detonated in his brain pan. He fell without a sound, blades slipping into the mud from his lifeless fingers. The warrior had been almost obsessive in his training. That had counted for nothing at the last. It was an ignoble end.
 
The Purge is primarily a character creation story for a minor figure in Reynolds’ Word Bearers’ trilogy (which does not fit the point of The Silent War at all... lol). We are told a lot of important things about Sor Talgron, but mostly he just hangs out while others do the work.

There’s an additional “surprise” left on Terra but it seems like no other author picked up on it for the Siege of Terra series, unless they enter the story very very late.

As for the primarchs, Rogal Dorn features, or at least a version of him that must have just suffered a traumatic brain injury.

It’s an enjoyable, combat heavy novella that also gives the Ultramarines a little bit of bite, but you’d lose nothing by browsing the wiki.