Preheat the oven to 375°. Pierce the baking potatoes and the sweet potato in 2 or 3 places and bake for 1 hour, or until tender. Let cool slightly.
Meanwhile, in a medium saucepan, bring the half-and-half to a simmer. Add the celery root and garlic and cook over moderately low heat until the celery root is tender and the half-and-half has reduced to 1/2 cup, about 30 minutes.
Transfer the contents of the saucepan to a food processor. Pell the sweet potato, add it to the processor and puree until smooth. Scrape the potato puree into a bowl.
Halve the baking potatoes lengthwise and scoop out the flesh, leaving 1/4-inch-thick shells. Arrange the potato shells in a baking dish. Lightly brush the shells inside and out with the melted butter. Pass the potato flesh through a ricer into the bowl with the potato puree. Stir in the remaining 4 tablespoons of butter and beat with an electric mixer until light and fluffy. Season with salt and pepper. Using a spoon or a pastry bag fitted with a large star tip, fill the potato shells with the puree.
Preheat the oven to 375°. Bake the potatoes for about 1 hour, or until the filling is heated through and browned on top and the shells are crisp.
Make Ahead: The recipe can be prepared through Step 4 and refrigerated overnight. Bring the potatoes to room temperature before baking.