It certainly tests the adage that one should write about what one knows. Mr. Follett, a self-proclaimed Non-religious fellow, became quite intrigued by medieval cathedral architecture as a younger man. This bit of a hobby features as a central plot point in Pillars, with a good deal of the story revolving around the construction (re-construction actually) of a cathedral during the reign of King Stephen. More correctly, perhaps, during the Civil War between Stephen and Maud.
Is it the best piece of writing he's ever done? I'm afraid I can't answer that question, at least not yet. I can't pull out of recollection anything I've read that he's written. I do read spy action-adventure mystery thrillers, but not frequently. It is a well-written tale, which I enjoyed and recommend.