I have not studied Helper or his book, but my understanding is the book was an economic criticism of slavery and an argument for its end on that basis. By the time he published, the slavery controversy had become a complete lightning rod such that criticism of slavery was considered by many to be criticism of the South; the mere possession of his book was actually outlawed in some places.
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As usual, you're on the mark.
In addition to being a very competent economist w/o a political ax to grind, Helper was clairvoyant about trade relations between Britain and the South.
As the Civil War progressed, the British realized that the South would lose; as such they would need a new supplier of raw cotton for their Mills yet they didn't have to look very far.
Baron Clive of Plassey was the first British Administrator of (East) Bengal/the Indian Raj; around the mid-18th century.
He had advised earlier Parliaments that the climate, geography and sea access of Bengal
to the British mainland was ideal for the production of raw cotton.
So when our Civil War ended, the British simply sent their ships to India rather than our Carolinas!