At the start of this year I read a book called "The Lucky One", I dont really remember it much in depth, but enough to relate it back to "The Line". The lucky One is a story about a soldier, who went to war in Afghanistan (I think - it was a war somewhere around there), in his travels with war he found a picture of a girl, he picked it up and got people to ask around to see if anyone had lost a photo. To his surprise, he noticed that he became extremely lucky when he had the photo with him. He won money when he played betting games with other soldiers, he escaped many very near death experiences. So when he come back home from war, he decided to try and find the girl in the photo. Amazingly he found her, with basically no information about her at all. he talked to her for a while, and she gave him a job at her dog training school. Cut a long story short, they fell in love. He didn't tell her about the photo, and soon enough she found out. she got a bad impression, and they had a big fight. he tried to explain to her, that the photo was lucky, but she didn't believe him. After her calming down, she talked to her nanna, and her nanna told her that many strange things happen in war, and you just don't know how it causes people to react. "Perhaps he did feel lucky when he had your photo" she said
She told her that you just don't know how war can change people, and make them believe different things, if you don't go to war you will never understand what they go through. After a while her nanna convinced her to give him a second chance.

I think the nanna, relates the book back to the line when she tells Beth* that you never know what the war can do to people, and how it can change them.