The Heart of a Witch — Judith Hawkes

Since childhood, twins Kip and Shelley Davies have been fascinated by the elegant inn across the street and the mysterious guests who return again and again. Now teenagers, they begin working there -- and are drawn into a world they never dreamed could exist. Ignoring the hints of danger, they embrace a new existence of secret ceremonies and tantalizing powers, until their lives become a dream from which they may never awake.