Found it. "Eight Sabbats for Witches" by Janet & Stewart Farrar. Copyright 1981.
A representative passage from the book:
"When the High Priest reaches To the wonder and glory of all men in the Invocation, he stops. The Maiden then fetches her athame from the altar and ritually opens a gateway in the Circle by the door of the room. The coven file through and leave the room. The Maiden steps last out of the Circle, ritually seals the gateway behind her, lays her athame on the floor outside the Circle and leaves the room, closing the door behind her. The High Priest and High Priestess are thus left alone in the room and the Circle...No member of the coven may question them on it afterwards, directly or indirectly.
An amusing aspect is their little hints about doing the rituals in the modern world. Apparently birthday candles are excellent for making a crown of candles. Also, if you light a bonfire in the woods for your ritual, keep an extinguisher handy.