The Folk Keeper, by Frannie Billingsley, 1999

Corin, nee Corinna, has learned to survive in the Foundling Home by turning herself into a boy and extorting the secrets of Folk-Keeping from other Cellar-bound keepers. Her cover is almost blown at 15, when an ailing lord summons her by name to his manor, breathing his last breath in her ear. A fantasy at once bloodthirsty, mysterious, and romantic; a well-rewarded read, if at times confusing. Strongly recommended.