it would have been without their input.の仮定法がわかりません
For their meticulous, insightful and helpful comments on the manuscript. Their eye for detail was truly impressive. Each brought their special, but different, strengths to bear, making this a better book than it would have been without their input.
回答
Each brought their special, but different, strengths to bear, making this a better book than it would have been without their input.
つまり、
Each brought their special, but different, strengths to bear, making this a better book. The book would not have been a better book without their input.
ということを言っています。
この質問に回答する
For their meticulous, insightful and helpful comments on the manuscript. Their eye for detail was truly impressive. Each brought their special, but different, strengths to bear, making this a better book than it would have been without their input.