Of course, artificial scarcity. Most likely wherever you wanted to buy it don't have a licence to generate additional copies, are just allowed to sell some amount. Alternatively it can be even lower blow, abc of speculation, create demand (for example by bribing schools/teachers/boards/minister to require their product as official/recommended textbook), and then at some point withdraw it from sale and put it after a time with higher prices.

It works as good with physical and digital goods, though the second one require more consumerist conditioning of population.