Three years ago, Andrew Carroll embarked on a worldwide journey to seek out the most powerful and unforgettable letters ever written during U.S. wars, from the American Revolution of 1776 to the fighting now going on in Afghanistan and Iraq. Out of the tens of thousands he collected, the author chose 200 letters and e-mails for his new book, Behind the Lines.

The world's battlefields have changed over the centuries, but not the soldiers' need to write to their loved ones back home. Surprisingly, Andrew Carroll says, between bullets and bombs? soldiers find the time to write, and write eloquently. "From combat letters to love letters, to letters to their children. They don't do it consciously," he says. "They just tell their stories, but because of what they experience and who they are, their stories are so compelling and so gripping and this is what we're trying to preserve."

Mr. Carroll says some of the letter writers represented in Behind the Lines preserved their experiences in novels, such as a young GI taken prisoner by the Germans in World War II. "He was writing home to his dad about his experiences of being captured and (being) a prisoner-of-war and then his liberation," Mr. Carroll says. "It just happens to be a 21-year-old Kurt Vonnegut Jr., who of course went on to become one of America's most respected writers and wrote this book, Slaughter-Five about the fire-bombing of Dresden and he was there when it happened."

Another set of letters dating back to the same era, Mr. Carroll says, belongs to the late Marine Pilot Harry Kipp, and his wife, Norma. "It's a woman in World War II who wrote to a Marine just offering her support," he says. "She never met him, did not know him. She was a friend of a friend just saying that she was thinking of him and appreciated his service."

Norma was only 16 years old when she received a reply from Harry, the first of more than 100 letters he wrote to her. "I was shocked to get this airmail letter from a captain in the Marine Corps," she says. "I was an honor student in English in high school, and I was very happy to read such a legible letter with correct margins, punctuation and spelling. I sort of analyzed the letter from that point of view."

But soon, Norma says, they started to feel very close to each other. In his 8th letter, Harry proposed to her, even before they met in person. She wrote back? "This is what I wrote him on August 27th, 1944," she says. "'My dearest Harry, Yes, yes, yes, I will marry you! Just as soon as you can come for me. I want more than anything in the world to be your wife, forever. I love you with all my heart. Your Norma.'"

Thanks to their correspondence, Norma says by the time she and Harry got married they felt they knew each other very well. He said in one of the letters, "I can hardly realize that so very soon, the dearest, sweetest, loveliest, little darling in all the world will be my wife. We've waited nearly 2 years now, and if we do find ourselves a bit embarrassed when we meet, it will be no more that it's natural to expect and the awkwardness will disappear in a moment, because deep in our hearts we understand each other, and we know that our 2 individual lives are bound to each other inseparably and nothing under the sun can ever change that.'"

Sometimes, what a soldier doesn't say reveals more than what he does. Behind the Lines author Andrew Carroll points to 2 letters written by a Marine in the 1990 Gulf War. "In the first letter he writes to his mom, saying 'Everything's fine. The war has not started, if it ever does. And I'm in safety. Don't worry about me, I'm not in any danger.' Just after that," Mr. Carroll says, "[the Marine] wrote a letter to his brother saying 'Here is the real deal' and he wrote it in code. And it's essentially that 'the war is imminent and I'd be at the tip of the spear, very much in danger.' I've never thought of such a letter written in code like that."

While today's warriors have access to cell phones and the Internet, Andrew Carroll says there's still something special about a message on a piece of paper. "The language certainly changes," he says. "It's more formal in the past letters, and especially e-mails are more conversational today. When I went to Iraq and Afghanistan to talk with the troops during my global search for these letters, many of them said to me, 'Of course we use e-mail all the time. We can call home in certain moments, but when we want to write something really meaningful we still put pen on paper.'"

The author says he hopes letters exchanged between fighters in the battle zones and their families at home will help reveal how war affects each individual. He says his book, Behind the Lines, is part of a larger effort to preserve correspondence that's not just personal, but insightful and historically significant.