Even mormon Apostle B.H. Roberts conceeds that one of the other writings in existance at the time, Ethan Smith's View of the Hebrews furnished structural material for the bom. Roberts admited that the evidence pointed in this direction:
It has been pointed out in these pages that there are many things in the former book that might well have suggested many major things in the other. Not a few things merely, one or two, or half dozen, but many; and it is this fact of many things of similarity and the cumulative force of them that makes them so serious a menace to Joseph Smith's story of the Book of Mormon's origin ... The material in Ethan Smith's book is of a character and quantity to make a ground plan for the Book of Mormon (B. H. Roberts, Studies of the Book of Mormon, 2nd edition (Salt Lake City: Signature Books, 1992, p. 240).
In this case, it is a mormon with integrity who points to the clear evidence that smith et.al. came up with the material for the bom from existing sources, and not a mythical set of golden plates (that mormon history itself indicates were not used).
What he wrote was a work explaining how some geography theories as well as books like View of the Hebrews could lead to critics (like yourself) in denigrating the Church. He wrote this for the Church leadership giving them a heads up.
Some Anti Mormons actual dredge this up from time to time, (like today) claiming he lost his faith in the Book of Mormon. Funny stuff.
“Let me say once and for all, so as to avoid what might otherwise call for repeated explanation, that what is herein set forth does not represent any conclusions of mine. This report [is] ... for the information of those who ought to know everything about it pro and con, as well that which has been produced against it as that which may be produced against it. I am taking the position that our faith is not only unshaken but unshakeable in the Book of Mormon, and therefore we can look without fear upon all that can be said against it.”