This was... an interesting one.
To overly simplify things: this book is those two friends who don't actually like each other taken to its absolute extreme.
It started off promising. From the very beginning, I believe it did a good job of portraying the kind of casual 'friendship' most people experience at some point in their lives where, whether by proximity, industry, blood, or some sort of external influence you maintain a relationship with someone that, looking at it clearly, you don't particularly like. You don't get each other's humor, not understand or take the things each other say the wrong way and find that you feel a little drained after talking to them. You don't overly DISLIKE them, you're sure they're a fine person, but you are just operating on different wavelengths and you don't ever see yourself becoming truly close to them.
In the first pages of the book there's little needling here and there, aloud for Doug and mentally for Bob. Doug is arrogant and controlling and Bob is whiny and dismissive of Doug's genuine expertise, they both initially suck so equally that were it not for the insight of a POV you could wonder who's actually gonna end up the bad guy here. But as we go on that becomes less and less of a problem as Doug gets progressively more irritable, more insulting and more of his resentment for Bob's "perfect life" spills out. I won't outline the entire trajectory of the story, but I felt like the first 2/3rds of the book did a good job of portraying the degradation of an already flimsy relationship, and the resentment brewing between essentially peers of vastly different levels of success within the same or similar career trajectories.
Then within a few pages the facade totally drops, and I believe the book falls off and hard. Bob and Doug have a conversation regarding spirituality and the afterlife - a sore spot for Doug whose son Artie had killed himself some time previously. This sore spot starts to bleed profusely when Bob outlines his world and afterlife view as essentially being the plot to Matheson's other novel, "What Dreams May Come", where a soul's life and afterlife is subject to a karmic penance system, meaning that your hardships in this life may very well be a reflection of misdeeds in a past one.
Well, Doug really very much doesn't like this, as a recurring pattern with Doug is his utter inability to take any measure of accountability for the issues in own life, such as his divorce being a direct result of his admitted infidelity, emotional neglect and probable abuse; his son's descent into drug addiction and ultimately suicide as well as total estrangement from his daughter due to being an emotionally neglectful parent, his failing acting career due to his extreme arrogance, etc. The story multiple times points to this being a form of 'malignant narcissm', which as someone with no formal education in psychology, yeah sure I guess. But ultimately being presented with a viewpoint from a man he has an inferiority complex towards that his struggles are ultimately a result of his own faults both on a physical and metaphysical level - as someone utterly unable to find fault within himself... Well that tips him over the edge.
His aggression and thinly veiled hostility turns into outright malice, and anything Bob does or says from that point whether retaliatory or placating has the same effect, because Doug is well and truly off his rocker.
Within a few pages Doug is violently raping Bob which, while I don't believe sexual violence should be too taboo to include in forms of media where gratuitous violence is practically celebrated, I do believe it should be done with some tact and actually be necessary within the story, and here it's just not. It's the first major example of Doug's heel turn from believable kind of shitty person who snapped to full blown cartoon villain. There are some homo-erotic subtones from Doug throughout the story, and you could make the argument there's a slight 70's-macho-man-with-repressed-homosexual-urges thing going on, but ultimately it leads to nothing and this still comes out of left field. The book could have completely done away with it I think the benefit of the story overall.
For the rest of the book, Doug commits to his villainy act so thoroughly it ceases to be tense and just comes across as goofy. He's leaving him notes and food, calling him babe and truly stalking him like he's LARPing a billionaire poor people hunt. On more than one occasion, Bob is actually successful in getting the upper hand on him, but restrains himself because he's, "not like him", pulling the tired "if you kill a killer, the amount of killers stays the same" trope that as a MCU and DCU fan I am very tired of. Besides occasional off-putting interruptions from Doug, there's a decent 'man conquering himself" thing going on with Bob until he finally reaches their final goal, Doug's cabin, only to find that Doug had beaten him there and was chumming it up with Bob's wife who Doug also greatly coveted. There's some creepiness, Doug once again flipping from normal to unhinged to once again demonstrate the depths of his sociopathy, and another scuffle which Bob and Marion come out on-top of, with Marion adding one final, "you're not him" to stay Bob's wrathful hand. At least this one made sense as Doug was finally well and truly defeated at this point, and though deserving of his fate, it wouldn't actually help anything like it would have previously - though it sure would have been satisfying.
Ultimately the book ends with Doug strapped to the hood of Bob's Bronco like a fucking hunting trophy to drive until he either he reached the police or they found him... what?
If I were to rate this book individually based pre vs post Doug snapping, it'd be a 4 and a 2 respectively. If it had kept building on what the first 2/3rds was showing rather than having Game of Thrones season 8 worthy heel turn, it would have sat better with me. But it is what it is and we got what we got. If you take anything from my wordy, unprofessional and disjointed review, it's that while it's not a waste of a read, Matheson has some truly great works, and unfortunately this is not one of them.