Hours before a big Giants-Cowboys showdown last December, then-Cowboys coach Bill Parcells sat in a car outside Giants Stadium and made it clear to the Giants that he was interested in their soon-to-be-vacated general manager's job. That's one of many surprising revelations in "The GM: The Inside Story of a Dream Job and the Nightmares That Go With It," by Tom Callahan (Crown Publishing) - a story about the 2006 Giants told with the help of retiring GM Ernie Accorsi. A copy of the book, due to be released on Sept.18, was obtained by the Daily News. According to the book, six hours before the Giants and Cowboys battled on Dec. 3 for first place in the NFC East, Parcells "sat with a former Giants colleague in a car in the parking lot....What Bill supposedly said became the lead item on the front office grapevine: 'I'm available to be the Giants' GM if they get stuck."