Because the initiatory contents are closed, the historical work has been done on the material that entered the public record by other routes: Gerald Gardner's own drafts and papers, which passed into collections and were studied there, and the various texts published from the 1970s onward by people who chose to break the restriction. Working from those, historians of modern paganism have been able to describe how the book was assembled without needing access to any coven's copy. The findings are now broadly agreed. The earliest layers draw on identifiable printed sources of the early twentieth century and before, including ceremonial magical literature then in circulation, and the material was substantially rewritten in the 1950s by Doreen Valiente, who removed much of what she recognised as borrowed and replaced it with verse of her own. The claim of unbroken transmission from a surviving witch cult, made when the book first became public, is not supported by that evidence.
None of this settles what the book means to the people who keep one, which is a different question and not a historical one. The atlas records the scholarly account of the book's composition and the tradition's own account of its authority as two separate things, because they answer different questions and are not in competition on the same ground.