This may be the worst book I've ever read. A friend of mine bought it for me to take on an 8 day backpacking trip because the events take place on a backpacking trip - 'field research' for a novel the protagonist is to write.
The writing is simply horrible - I've read other works by Matheson but this one seems as if it were churned out as a grammar school writing assignment and penned by a rather dim student. The plot, boiled down to a single sentence, might sound suspenseful but the execution is so extraordinarily bad (and not, unfortunately, in a bad way in which you can find humor) that reading the book is sheer mental torture - having this book as my only reading material almost made me want to cancel my trek the first night out.
As other reviews here have offered a synopsis (and - shudder - praise), I'll not go into an actual review of the book. But one example of a semi-major theme - and how terribly it's handled - might help to illustrate just how bad the book is: the main character knows nothing about backpacking (or, it seems, anything having to do with flora or fauna beyond his driveway or that found in restaurants) but, for some reason, he's writing a book in which backpacking is the theme. He's deathly afraid of encountering bears, snakes, coyote, mountain lions, (and, one would assume, squirrels), etc, on this trip so, naturally, he encounters more bears, snakes, coyote, mountain lions, etc., on a three night trip than one would normally encounter in three months in the wild. And although he's quivering in his boots at each encounter he finds that he has a sort of mystical communion with these creatures due, we're led to believe, to his highly developed spiritual awareness and general good karma.
Generally I'd say that a book this bad demands to be read - but, in this case, it's such a painful read that it should be reserved for those with only the most uncontrollable masochistic tendencies.