A very interesting article about early conjuring, conjurers, books and manuscripts. Includes detailed research on how the term 'hocus pocus' came about.
Up to about the end of the sixteenth century, the wonderful was by preference regarded as magical—as the work of supernatural powers, good or bad, but mostly bad. A puzzling phenomenon, the explanation of which was not obvious, was generally regarded as due to the invocation of powers above the natural. Many of the feats ascribed to demoniacal or divine aid are such as we know to be similar to the tricks included in the conjurer’s repertory. The performers who knew the modus operandi endeavoured to keep the secrets of their marvels, as modern conjurers are in some cases able to do, though with greater difficulty, and those who could explain had every reason not to do so. It answered their purpose, as a rule, to preserve a monopoly and, in many cases, to retain the reputation of possessing supernatural power, even when their tricks depended merely on some special apparatus, some more or less simple sleight of hand, or some mere trick of confederacy.