Reign of Quantity refers to the title of a book by Rene Guenon, the Sufi convert from Catholicism. It was written in the 1940s, a book that played a seminal role for many people in the rediscovery of "metaphysical" thought outside the mainstream intellectual culture (just at a time when the logical positivists and analytic philosophers were destroying its last traces in Oxford). I have mentioned it several times, not for the trivial reason that it influenced me, but because there are insights in it that remain important, not least for educators. An article has recently appeared in Sacred Web journal by Patrick Laude looking back at Guenon's book.
One may bracket out the historical theory about repeating cycles (further developed by Robert Bolton in The Order of the Ages), or Guenon's tendency to confuse logic with ontology or Islam with Vedanta, while still retaining the thought that in today's world we are seeing a grand reversal or inversion that places Matter over Form, "quantity" over "quality" – amounting already in many places to the apparent disappearance of quality altogether.
(Connections could be made here to Zen and the Art of Motorcycle Maintenance, which is all about the recovery of quality, but I restrain myself.) As our determination to measure everything grew with the Enlightenment, so did our capacity to control it – or rather the lure of controlling it – until every human art from education to economics, from architecture to agriculture, became reduced to a numbers game. Human achievement was assessed by GDP, extension of life, earnings, records broken, products bought....
And yet numbers themselves are qualities before they are quantities, as the Pythagoreans knew. The loss of the symbolic, analogical, religious, and musical dimension of number made possible this modern profanation. Number as quality relates things interiorly to each other, through self-identity, proportion, and mutual implication. The world coheres with itself in number. Each number is full of meaning because it connects with everything else, and ultimately with the One that is beyond number.
The sheer fun of numbers, and a great deal of the mystery (what are they? why do they work?) is captured in books by Clifford Pickover (including this one that calls Mathematics "the loom of God") that would make a good addition to any library. If it is the use made of numbers by medieval Christendom in particular that interests you, going back to the Greeks, one classic work is Medieval Number Symbolism by Vincent Foster Hopper (Dover).