To mention just one small example: common spellcasters might not be able to create or replicate spellbooks for several reasons. One might be that there is a defensive spell cast on the book which defends against replication; it might manifest itself in the same way that Hermione hexed the traitor who finked out Dumbledore’s Army in book 5.

Another method would involve a self-imposed geas invoked when purchasing the book, which would prevent the buyer from creating a copy. Naturally the geas would include clauses preventing a friend who “borrows” the book from making copies as well.

Finally there’s Terry Pratchett’s ideas about magik, which are deeply linked to modern physics. For example, in Discworld, magik-users recognize it is incredibly difficult to create something (such as a book) out of nothing when one considers the potential energy required to create even a small object. It’s e=mc2 with a vengance. A classic example is farie (or lepruchan) gold. It’s easy to create, but does not last. In other words, the spell subtitutes entropy for energy. A gold coin may be created, but only until the next sunrise. To make a real coin out of genuine gold requires a master magician and resources at least equal in expense to the gold coin itself. Creating gold by magik is, hence, inherently uneconomical.

If Ms. McArdle had bothered to consult Europe’s fables for (oh…) the past millenium, she might have caught that concept…