An early sci-fi novel that sets up the question more sharply than most later books in the genre. If you could prevent every disaster, would you, and what would the cost be of a humanity that never had to grow up. The book argues, quietly, that the absence of risk is its own kind of stagnation. It reads more pointedly now than when it was written.
I have always liked books about time and time travel, and this one is a good reminder of why.