John McVay knew his grandson, Sean, was serious about coaching when as a youngster he would fall asleep reading Bill Walsh’s book: “Building a Champion: On Football and the Making of the 49ers.” It was published in 1990. “It’s a game plan for anybody who wants to be in this business,” the elder McVay told The Post by telephone from his home near Sacramento, Calif. “Sean would always be reading it, and then he would tell you, ‘I read this and that and the other.’ He would read a few pages at night until he fell asleep. You could tell he had a deep interest in it.” That interest has turned into becoming the head coach of the Rams, who visit MetLife Stadium on Sunday to play the Giants. When hired in January, the then 30-year-old McVay was the youngest head coach ever in the NFL. He turned 31 a week later and has changed the culture of the Rams. They have won four their last five games and stand 5-2 on the season. “He did make his grandfather a liar because I said it’s going to take three years to turn it around,” John McVay said. “He’s made me a liar already. Nobody’s saying we’re going to the playoffs, but he’s got them going.” McVay, 86, will be watching on television Sunday, hoping his grandson improves to 6-2 with a victory over the 1-6 Giants, a franchise McVay coached for 2 ½ seasons in the late-1970s. His tenure included “The Fumble” game, which the Giants lost to the Eagles on Nov. 19, 1978, at Giants Stadium. A botched handoff from quarterback Joe Pisarcik to Larry Csonka was recovered by Herman Edwards and returned for a touchdown that gave Philadelphia a 19-17 win.