If you're looking at getting this book, you're most likely a fan of Warhammer and more particularly, a fan of the Iron Warriors and Perturabo. I would consider myself among them, having gotten my copy of Storm of Iron signed by Graham McNeil years and years ago. You want to see more of the Iron Warriors from a more empathetic perspective and gain insight into the legions and it's primarch. I won't say that Hammer of Olympia fails to do this completely, it's not a complete whiff, but it definitely falls short of expectations.
Perturabo is a complicated character- there seems to be a perpetual tug-of-war between the Black Library authors on just how to portray the character; either as a cold, calculating mind that turns to logic to suppress a wounded personality or a rampant narcissist that wants to be glorified but can't seem to grasp at how. Graham McNeil managed to strike a wonderful balance between the two in Angel Exterimatus, which was what got me interested enough in the character to pick up Hammer of Olympia. Guy Haley tries follow in McNeil's footsteps, but struggles with the execution.
This can largely boil down to the fact that most of the book has nothing to do with Perturabo; the book focuses on the campaign against the Hrud, a species of nomadic aliens whose biology causes temporal distortions in space and time. Reading the blurb on the back you would believe this to be the preface to the larger part of the story, Perturabo dealing with the rebellion of his homeworld, but in actuality it takes up the vast majority of the book- Perturabo's return to Olympia is a grand total of 28 pages out of the 218 or so total.
This isn't to say that Guy Haley didn't try- his ambitions are large for a 200 page book, as we do get snippets of Perturabo's life on Olympia growing up. The lore of 40k is a massive pool as deep as it is wide, so there's no shortage to cover, but given the size of the book, Haley spreads himself thin on each of the topics. They aren't poorly written, quite the contrary, but the real meat of the story; Perturabo's history and tortured relationship with hims homeworld and his subsequent sense of identity, feels rushed. Throughout most of the story Perturabo just comes across as an arrogant, egotistical monster with no regard for anything but making big Daddy Emperor happy, and then suddenly in the last 10 pages he experiences a whole book's arc. Haley understands the spirit of Perturabo and has good goals, but he didn't give himself the space or establishment it deserved in order to really hit home.
Now this isn't to say the book is without merit. Despite being of the opinion that the Hrud wind up being a subplot that grossly overstays it's welcome (intended to illustrate Perturabo's flawed approach to acting as a leader), Haley does a fun job depicting the aliens and the effects of their anomalous biology, turning people to dust or reverting their age and the ruin they bring purely by the fault of existing- the most enjoyable parts of the read focus on these often disastrous effects and are super creative; so much that the Hrud wound up being the most interesting part of a book intended to be about the primarch of the IV legion... And I guess that's really the core of the problem: the book isn't bad but it bites off far more than it can chew in it's limited page count. It needed a tighter focus.
Ultimately you can do a whole lot worse; Perturabo: Hammer of Olympia isn't a bad read, but if you were hoping to get a character study and origin story, you will be disappointed. Perturabo's history meeting the Emperor and his time being brought into the fold of the Imperium are barely touched and his growth as a character feels underdeveloped; flip flopping between depicting him as a cartoon villain and being a tortured anti-hero at the flip of a switch. Likewise, the decisive moment of Olympia's destruction and the fissures it causes in it's home legion (which will sow the seeds of the Horus heresy later on) are barely touched on.
Read Hammer of Olympia if you want to get some glimpses into Perturabo's homeplanet and history- but remember it's just that: glimpses. Nothing in depth. You should also read it if you like the Hrud, as now I want Hrud miniatures for the 40k tabletop game.
But if you're looking to see the human flaws and vulnerabilities of the Lord of Iron at flaws with his ambitions, you'd be better served picking up The Horus Heresy: Angel Exterminatus by Graham McNeil