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The Sun reports that the 32-year-old will release his book with publisher Random House.
"James just said 'F**k it', it was a lot of money and why not? Loads has happened in his life people don't know about," a friend told the paper.
"He spent a lot of time living in New York when he was in the movie The History Boys. They were great times."
They continued: "He might be 32 but there are a lot of stories to tell. He was an actor first and the fame came later. He's fiercely proud of his family and his humble upbringing and wants to get that across too."
The Gavin & Stacey star is currently expecting his first child with girlfriend Julia Carey.
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