Bravo Two Zero by Andy McNab

When eight members of the elite SAS regiment embark on a highly covert operation, they are each laden with 15 stones of equipment, needing to tab 20km across the desert to reach their objective. But within days, their location is compromised. They engage in a fierce battle. They escape on foot to the Syrian border. Three men die. One escapes.
But four men are captured. For them, the worst is yet to come. Delivered to Baghdad, they are tortured with a savagery for which not even their intensive SAS training has prepared them...