The best that can be said about Kurai's novel is that it is entertaining, one of those page turner affairs that can be read on the beach, while drinking a coffee... Because from the very beginning this is silly as they come (and, yes, I had watched Miike's movie, so I knew what I was getting into): a psychopath is attacked by a person with a monster's mask (the lumberjack monster of the title) and gets injured. The injury? A chip in his brain is broken and he is no longer a psychopath. Will he change, will he stay the same, will he...? Really, it is super stupid, from the motivations of the 'bad one', to our main character's behavior (and his friend, the killer doctor). And then, you have the police, totally pointless and just there so the novel becomes a novel (if not it would barely be a novella). And the unnecessary time jumps, characters doing plot exposition, things that happen just too casually ( ).
If you leave your brain on the cover (pun not intended) you may 'enjoy' it.
The best: one of those easy reads
The worst: it is silly as they come, it has so much filling it could be a turkey, so many unnecessary characters