Ah celeriac, troll of the root vegetables. You are a gnarly thing. What is so funny is that celeriac is one of the most well behaved plants I have ever grown. I planted a crop in early spring, and harvested them this past week. The roots would have gotten larger I am sure, but I needed the space they were in to plant some greens. All spring and summer, these lovely upright fountains slowly grew- I nearly forgot about them! They got no fertilizer, were not marred by pests, never drooped and now have graced us with our first root vegetable of the season. In case you were wondering, I recommend growing them. You may have to start seeds indoors, but these lovely plants can go out rather early in the spring.
So far, I have made only a cream of potato and celeriac soup with my new favorite root, though I am sure you could treat it as any root vegetable. (If you want to try, follow my potato and fennel soup recipe, substituting celeriac for fennel.) Just cut away the thinner, wild looking roots as well as the outer layer, and roast, bake, puree away! I even saw a recipe on a raw food website for celeriac 'risotto'. Celeriac is related to celery, so it has a faint, though sweeter resemblance in taste. I am not aware of anyone selling these little monsters at the markets, but if you ask, maybe someone will put it in their plans for spring.