Make mop: Melt butter in a nonreactive saucepan over medium heat. Stir in cider, bourbon and soy sauce. Keep warm until ready to use.
Make rub: Place salt, brown sugar, paprika, pepper, mustard, garlic powder and celery seed in a small bowl and mix with your fingers, reaking up any lumps.
Prepare ribs: Place a rack of ribs meat side down on baking sheet. Remove the thin papery membrane from back by inserting a slender implement, such as a butter knife or tip of a meat thermometer, under it. The best place to start is on one of the middle bones. Using a dishcloth, paper towel or pliers to get a secure grip, peel off the membrane. Repeat with remaining rack.
Set aside 1 tablespoon of rub for serving. Sprinkle remaining rub over both sides of ribs, rubbing it onto meat. Cover ribs with plastic wrap and refrigerate them while you set up the grill.
Set up grill for indirect grilling and adjust cooking temp to medium (325ºF-350º). Place large drip pan under ribs.
When ready to cook, brush and oil grill grate. Place ribs, bone side down, on grate over drip pan and away from heat. Cover grill and cook ribs 45 minutes.
Mop ribs on both sides with mop sauce. Re-cover grill and continue cooking ribs until well browned, cooked through and tender enough to pull apart with your fingers, 45 minutes to 1 hour longer, 1¼ to 1½ hours in all. When ribs are cooked, the meat will have shrunk back from the ends of the bones by about ¼-inch. Mop ribs again every 15 minutes. Replinish coals as needed.
Just before serving, brush ribs on both sides with your favorite barbecue sauce and move them directly over the fire. Gril ribs until the barbecue sauce is browned and bubbling, 1 to 3 minutes per side.
Transfer ribs to cutting board. Let rest a few minutes, then cut in half. Serve at once with barbecue sauce and reserved rub.