Like many 70's science fiction novels, the premise itself is pure fantasy. There's no way for someone's dream to change the world retroactively, yet these dreams do. However, these dreams can't change the rules of the universe, they can only change the course of events. Once the changes have happened, what remains fits well within science fiction.
This book is essentially a time travel novel without time travel. It contains all the tropes of that sub-genre: messing with time brings with it vast moral implications, and the results of messing with time are quite unpredictable, and worse, come at a higher cost than you'd expect.
If there's any message to be found here, it's that you can fix the world because you can't fix humanity.
I found the overall level of writing quite engaging, with appropriate descriptions that created the setting without overburdening me with detail, conversations that stayed well inside the narrative, and a tight use of all the major characters. Overall, I'd describe the work as minimalistic, using just enough narrative to get the story across. By the time that you reach the end, even though the book is short, you're emotionally ready for the end. The novel has done its job.