LDS Polygamy and Joseph Smith Historian Don Bradley explains that there are two ways to treat historical sources.

Don Bradley's Academia.edu page, with several of his publications, including one on the John Alger account of the First Vision:
https://josephsmithpapers.academia.ed...

Don's book The Lost 116 Pages: Reconstructing the Book of Mormon's Missing Stories:
https://www.amazon.com/Lost116Pages...