Two weeks ago, Omnivore Books on Food quietly opened its doors in Noe Valley. When I found out about it from a friend of mine who is much hipper than I am, I nearly wet myself with joy. I have been known to lose myself in used bookstores for hours, but I have never been to one dealing exclusively in cookbooks. Housed, appropriately enough, in a former butcher shop, Omnivore is the dream child of Celia Sack, an antiquarian book dealer with a special passion for cookbooks. Even her name sounds as though it came straight from a novel. Celia Sack. It is, to me, a name that should be attached to a book store.