Damian favourite book is called "Six O'Clock Saints". Popular in the UK in the 1950s, it is surprising that any parent would give a copy to their child, as the screenwriter Frank Cottrell Boyce points out at 03:08 in the DVD commentary, since it contains all the gruesome stories that Damian tells in class, plus many more. Its inclusion is a sort of homage to Martin Scorsese, who, according to Boyce, has cited it in interviews as one of his favorite books growing up and that it gave him a wider understanding of the human experience than had been revealed to him as a child. Roger Ebert's 18 March 2005 review of the film, mentions that Boyce "got the inspiration for the screenplay from an interview in which Martin Scorsese said he was reading the lives of the saints."
The scene between Damien and St. Peter was not in the original screenplay. Director Danny Boyle mentions at 46:03 in the DVD commentary that he suggested Frank Cottrell Boyce re-write the screenplay as a novel to which that scene was then added. When Boyle read the novel, he decided the scene needed to be in the film because of the emphasis that the saints' appearances had gained in rewrites of the original script, so the scene was re-written in script form and shot as a pickup.
The trivia item below may give away important plot points.
At 38:37 in the DVD commentary, Frank Cottrell Boyce says "On the nature of this heist, where the guys rush the train and raid the train and actually it turns out that that's not the robbery, it's just a distraction, just a way of planting someone in that 'house'. You came up with that idea from Northern Ireland." And Danny Boyle responds "That's right. I heard this story about the troops raiding streets in the Falls Road in Belfast. It was just a distraction in order to drop a couple of men in the attics of the houses for a couple of days, out of contact and just simply eavesdropping, and then they would raid the street again and take the men out and nobody knew that they'd been there."