Threshold

From the author's website:

Threshold is one of my favourites of my books. Fantasy, but completely different from The Axis Trilogy or The Wayfarer Redemption.

Threshold is middle-eastern rather than medieval ... or perhaps medieval middle-eastern. Yes, that's it. It is not a heroic fantasy in the same sense that the Axis books are, and it doesn't follow the same fantasy formula that I used there.

It is the story of Ashdod, a land where mathematician Magi hold sway. The Magi worship the number One, as the number from which all other numbers emanate, and into which all other numbers eventually collapse. In a sense, then, the number One represents immortality - or Infinity (yes, you guessed it, I've based much of this on sacred Pythagorean mathematics). Several generations before the events of the book, the Magi had conceived of the perfect mathematical formula which will enable them to touch, and eventually step into, Infinity. In essence, to merge with the One.

This mathematical formula is expressed as a building, Threshold, with the Infinity Chamber at its heart. Threshold is a pyramid (unfortunate to use yet again the pyramid, but I must because of the pyramid's mathematical properties) made of glass, and most of the prime characters, apart from the Magi themselves, are glass workers, slaves on the construction site.

Threshold is told in the first person through the eyes of one of the glass workers, Tirzah. We learn of her very peculiar relationship with the glass, and the danger this places her in with the Magi. With Tirzah, we come to the realization that there is something very seriously wrong with Threshold, and that the Magi are not able to control the ways in which the formula is warping. Eventually, Threshold transforms into something that no-one, Magus or glass worker, can control. Threshold was supposed to be a bridge, a bridge to enable the Magi to merge with Infinity and the One. Instead, something comes across the bridge from the other side ... from Infinity.