If there was a redeeming factor, it came at the end, which was touching, and maybe even explains some of the unrealistic behavior throughout the book. But I don't think two nicely written pages at the end of the book justifies two-hundred-and-seventy pages of tiresome story. It would have made a much better short story; it would have been tighter, less tedious.
The first person narration of the book, as told into a tape recorder, gives a really cool voice to the first part. The first half, without giving away the plot, is a strange romantic obsession tale, written in a voice reminiscent of Edgar Allen Poe.
Matheson says that the first section of the book was written while at the Hotel in question and you can believe it. It is truly hypnotic and superior to the movie. The 2nd half is quite a bit less, but the power of the opening still resonates in your mind.
If you are interested in this book because you enjoy the movie, read only the pre-time travel bit. It we make the film experience richer. But don't bother with the 2nd bit.