Reviews of this book run the gamut from fantastic to awful. I think the reason why is because this is a story that operates on multiple levels.
First, and most obvious, it's a rags to riches story about a nondescript young man whose hard work, despite lack of education, vision and willingness to take a risk results in magnificent architectural feats - the American Dream, so to speak. The operative word, however, is Dream. Those readers caught up in the economics story will find the ending unsatisfying because Martin is a dreamer, not necessarily a Rockefeller, Carnegie or Gates.
The story as a dream is also effective - there is symbolism, interpretation, wild ideas that don't always make sense (including the sleepy wife Caroline).
In a minor way, it's also a walk through historic, pre-subway New York, and the descriptions of the city as it would have been at that time are fun to imagine.
It's an unusual, unique story that doesn't necessarily fit the classic novel style. Read it with no preconceived notions.