The premise is Jim Raynor is a 19 year old demolition derby racer and trucker who works a third job on his parent's farm. He's eager to do anything in his life other than be a farmer and gets perhaps a bit too excited about the offer from a military recruiter who visits him in a mecha suit. Signing up for the bonus, he ignores the fact the war against an organization called the Kel-Morian Combine is going badly for the Confederacy of Man.
Simultaneously, we get the perspective of Tychus Finnley (another character from Starcraft) who is a Sergeant in the Marine Core as well as professional criminal. Tychus is only in the military to make a buck and contrasts nicely with the idealistic Raynor. Rounding up the trio is Arik Bennet who is the rich son of a Confederacy family who gets himself drugged and sent off to war in one of those Vietnam urban legends brought to life in a star sector far far away.
This book is set before the games when there's a war between humans and aliens. Instead, it is entirely about its protagonists being caught up in a pointless war between two corrupt societies. The Confederacy is using its soldiers as cannon fodder while the Kel-Morian Combine are a depraved bunch of bandits masquerading as a military. Much of the book focuses on the growing sense of disillusionment from Raynor as he witnesses his side continually misuse the men serving them as well as how corrupt his immediate superiors are.
Later in the book, we get introduced to the sole bit of estrogen in the story with Cassidy a.k.a "Doc." She's a drug addict blackmailed into continual compliance by said corrupt superiors and ends up becoming a snitch on the team. I actually liked Cassidy as she was a character who was clearly caught between competing forces for her loyalty. The fact Raynor and the rest of the team aren't exactly a bunch of cub scouts as the war drags on is a matter which helps keep her loyalty suspect. As much as we like Tychus, he's not the kind of guy who inspires much loyalty or sense of love.
My favorite character turned out to be Bennet (going under the pseudonym Kydd for much of the book). He's a man who grew up in a rich family but finds himself ill-suited for the life and discovers that he actually is good as a military sniper. Unfortunately, Bennet sits on the fence on whether he wants to go back to his old life or not until events catch up to him. His choice is also one which is made on bad information and arguably didn't have to be made at all.
In conclusion, I really enjoyed Heaven's Devils and think it's not just a great Starcraft novel but a great piece of military science fiction in general. All the characters have interesting arcs and manage to develop in interesting but believable ways. The Kel-Morian Combine is treated as somewhat cartoonishly evil but the setting is a corrupt and decadent one. I recommend getting the audiobook version over the physical or ebook copies because Neil Kaplan (Tychus' voice in the games) does an amazing job.