What Fire Cannot Burn is the sequel to John Ridley's novel, Those Who Walk In Darkness. Honestly, I'm not sure there was much room for a continuation of the storyline. Soledad O'Roarke proved herself to be completely irredeemable and there's no indication that anything would be able to persuade her from her mindless hatred of superhumans. Worse, the world itself is too dark to be enjoyable.
The public of the setting are almost as bad as its Nazi-like protagonist. They voluntarily consented to stripping superhumans of all rights and treating them to summary execution if discovered. There's a few nods towards people who are in favor of increased superhuman rights but it seems too little, too late for me to care about this planet's population. It's too similar to the real world during the darkest periods of our history.
Perhaps recognizing that he needs to be slightly lighter than the book's darker than dark predecessor, John Ridley brings supporting cast member Eddi Aoki to the forefront. Eddi is a character much like Soledad but doesn't possess her same mindless devotion to the destruction of superhumans.
Unfortunately, Eddie is a lighter shade of black rather than a genuine hero or even antihero. For the majority of the novel, she's still a brutal violent thug who commits hate crimes as part of the world's anti-superhuman police force. It's only by comparison to Soledad that she comes off as an angel.
The plot of the book, unfortunately, struggles against the relentless bleakness of the original novel. A vigilante, seemingly superhuman himself, is killing all of the superhumans he finds. The problem with the premise is that Soledad and Eddi, herself, are largely ambivalent to the act. The world is so anti-superhuman a man running around murdering them elicits roughly the same sort of outrage Dirty Harry had for the vigilantes in Magnum Force during the first half of the movie.
I.e. none at all.
Eddi, eventually, goes through some mild character development at the end of the novel but it is almost an afterthought when the rest of the book might be devoted to her discovery of the basic humanity of her targets. Indeed, she's largely unphased by the innocent victims of her crusade against superhumans to an outstanding degree. Yet, surprisingly, she's still an improvement over Soledad.
In conclusion, What Fire Cannot Burn is a book far too dark and unpleasant for me to enjoy. It's lighter than its predecessor but still a bleak uncompromising vision of a Nazi-like Earth with superhumans filling in for other minorities. Frankly, the absence of a protagonist I can get behind just prevents me from enjoying the novel. That doesn't mean it's not slightly better than its predecessor, though. There's also a big twist I genuinely did not see coming and elevates the material somewhat.