Why the Spiral Staircase?

What is The Spiral Staircase? Maybe this will help to explain:

Armstrong owes the title of this book to Helen Gardner, who, in a lecture on Eliot's "Ash Wednesday" sequence, pointed out that the spiral staircase becomes a symbol for spiritual advance. As she began to immerse herself in sacred writings, Armstrong recognised that "the very absence I felt so acutely was paradoxically a presence in my life". Those seemingly arbitrary revolutions in her own progress had finally returned her to what she had been seeking all those years ago, "when I had packed my suitcase, entered my convent and set off to find God".

Review of "The Spiral Staircase", by Karen Armstrong