Tomorrow is the 90th anniversary of the Battle of the Somme, where on the first day of fighting July 1, 1916 40,000 were injured and 20,000 died.

One of those injured in the ensuing days was Harold Beechey. The seventh of eight brothers who were called up to fight, he had already seen terrible action, including places like Gallipoli. On this occasion, he was injured and sent home, but was eventually killed on April 10, 1917, at Bullecourt.

Harold lies in an unknown grave and would probably have remained a carved name on a memorial, but for two twists of fate. Firstly, he was the third of five brothers who all eventually died in this horrific war and secondly, his letters home from the Front, together with those of his seven brothers, survived. They enabled freelance journalist Michael Walsh to piece together an amazing book, Brothers in War. Not just a book about war, however, it's a moving historical document of a very ordinary British family.

He begins the book by writing a little snapshot of each brother, detailing what they did before they joined up. They saw action in different places including Gallipoli, the Somme, Flanders, the Mediterranean and East Africa and Michael interweaves their letters home from the front with accounts of the battles they fought in. "I didn't want it to be just a war book. I wanted it to be a social document, a human interest story," he said, when we met at his home in Ducklington.

Michael had the idea for the book after a bike ride to Great Rissington a few years ago, when he found a memorial to another family who lost five boys, the Souls brothers. Intrigued by the Souls memorial, he spent 18 months trying to find out more about them, but there was little left to find. "More than 80 years after the events, it was a mostly frustrating and unrewarding search," he writes in an epilogue to the book.

It was only after he'd written an article about them for Saga magazine that he was sent a letter mentioning the Beechey boys, who had grown up in a rural part of Lincolnshire, and eventually made contact with Josephine Warren, their niece. She invited him to her house in Devon to look at the material her mother Edie had saved, but which she had never looked at herself.

Michael explained his reaction upon first seeing the archive. "My heart was pounding," he said. "This little brown suitcase was sat in the middle of her dining table and it was just piled up with letters and papers." It must be every historian's dream to find such a rich archive, because Edie had also written a memoir of the family, which enriches much of the early part of the book.

In the book, you get a real feel for the brothers, some of whom wanted to fight, some who tried to avoid the inevitable call-up for as long as possible. "That comes from the letters," he explained. "What these boys were thinking at the time and what they went through and everything. It's all there in letters and words and voices that haven't been heard for 90 years."

Michael's interest in the whole period stems from the early 1970s when he was studying for his A-levels. For several years he went to the battlefields with a group of friends and read widely around the topic. He wrote the book before finding a publisher, but journalistic instinct told him it would sell. "I've got a good eye for a story and because it has all the things I'm interested in the family history, the First World War it just all came together."

What separates this book from so many others is the strong sense of family that comes through in the letters and the effect on the wider family as they receive news of each brother's death. It brings an emotional truth to the story that dispassionate military accounts often fail to achieve.

Michael, who is 52, has a strong sense of family himself. Married with two daughters, he spends Tuesdays looking after his 11-month-old granddaughter Poppy. He won't be there for her first birthday on Sunday, however, as he'll be in France for the memorial events. "Some things you just have to do," he said. "As I keep saying, Poppy isn't going to care if I'm not there." Anyway, he knows what's most important. "I shall be back for the Tuesday to do my shift."

Brothers in War is published by Ebury Press at £16.99.