Celeriac has a thick skin that comes off with a knife, not a peeler. Grated raw as a remoulade, mashed with potatoes, or in dice in a soup.
Heavy, firm, with no soft parts.
Cut the top and the bottom to get two flat sides, stand it flat, and remove the skin with a knife following the curve. Rub the flesh with lemon or put it in lemon water, otherwise it turns black.
Three minutes.
Whole, in the fridge, two to three weeks. Cut, the cut side under film, a week.
The page "Blended soup": celeriac and potato.