|I included the back cover here because it summarizes the book pretty well.|
Letter VII is the 7th of 8 letters that Oliver Cowdery wrote about the history of the Church. Joseph Smith helped write the letters and fully endorsed them on multiple occasions.
Letter VII declares as a fact that the final battles of the Nephites and Jaredites took place in the mile-wide valley west of the Hill Cumorah in New York.
This fact was well established during Joseph Smith's lifetime. It wasn't until around the 1920s that "scholars" began doubting what Joseph and Oliver said. They invented a so-called "Two Cumorahs" theory that claims the final battles took place somewhere in southern Mexico. (Others have claimed the battles took place in other parts of the world).
During Joseph's lifetime, Letter VII and the New York Cumorah were well known and universally accepted. As I explained in Moroni's America, I think the text and all available extrinsic evidence supports what Joseph and Oliver taught.
I think members of the Church everywhere should know what Joseph and Oliver taught, but LDS scholars have essentially suppressed Letter VII because it contradicts their Mesoamerican dogma. As more members of the Church read Letter VII for the first time, they come to understand for the first time what Joseph and Oliver actually said. They take another look at what they've been taught about Book of Mormon geography and realize that what they've suspected all along--that the Mesoamerican stuff doesn't make sense--is actually the case.
Everyone who reads the Book of Mormon knows it says nothing about jungles, tapirs, jaguars, huge stone pyramids, or Mayans. There is no way to reconcile the Arnold Friberg paintings of Central America with what we actually read in the text. Certainly, we don't read about Chichen Itza.
Many active LDS people have decided to live with the cognitive dissonance generated by the Mesoamerican and other non-New York Cumorah theories. Others have eliminated the cognitive dissonance by reading and accepting Letter VII.
But far more people--like 99+% in the world--will not live with the cognitive dissonance that comes with reading the Book of Mormon and being told that it describes a small group of Israelites who were absorbed into Mayan culture. It's difficult to imagine a greater impediment to missionary work than expecting investigators to believe this Mesoamerican theory, especially when they're also told that Joseph Smith and Oliver Cowdery told the truth about everything except the New York Cumorah.
In my view, Letter VII is the antidote to the ongoing nonsense about Mesoamerica.