Reader John Thompson, Reedsville, Pa., asks, “When are baseball announcers going to realize the difference between a ground-rule double and book-rule double.”A: Never. Much too late.Thompson’s correct. What’s almost always described as “a ground-rule double” — a batted ball bouncing over an outfield fence — actually is a “book-rule double,” in that the rule applies everywhere.Ground-rule doubles are a matter of individual park particulars and peculiarities — as Thompson notes by example, a ball that disappears in the ivy at Wrigley Field. That’s why, when the umps and managers gather at home before the game, they exchange lineups — and review that park’s ground rules.phil.mushnick@nypost.com