The members of Pastwatch use machines to observe and record human history: they can only observe: they cannot interfere.
Or can they? A novice to Pastwatch joins the team, and stumbles upon an event in history that suggests otherwise: a women from ancient history relates a mystical experience whereby observers in a far distant future watch her and her clan; indeed, she even describes the new member of Pastwatch, and talks as if to her personally.
Can changing Western history ensure the survival of humanity? What if Columbus had not been the pillaging, raping fiend he was? What if Columbus met Native North Americans who had already been converted to Christianity? What if Cortez had met an Aztec / Zapotec civilization that had been united instead of at war with their neighbors? If history can be changed, is it worth the astonishingly, inconceivable price that must be paid?
Pastwatch, by Orson Scott Card, is superb science fiction published by Tom Doherty Associates, in 1996. ISBN 0-812-50864-5 with 402 pages.
On the Shy David Book Review Scale from one to six stars (six being best), I think this book deserves six stars. This is one of the few books I've rated this high: I cannot write a book review well enough to give the reader an idea of how well the book is written. You "should" read the book and judge for yourself. If you like speculative science fiction, you will love this book.