In his "address", Whitmer attempts to reconcile two contradictory positions: belief in the divine inspiration of the Book of Mormon, and rejection of Joseph Smith. Since most people outside Mormondom believe that Smith wrote the Book of Mormon, this position is tantamount to, say, accepting the Quran while rejecting Mohammed! Whitmer thought otherwise, since the angel Moroni had supposedly showed him the original golden plates of the Book of Mormon. In his book, he argues that Joseph Smith was only commissioned to discover and translate the Book of Mormon, and that all later "revelations" from Smith's pen were fake.
I've reviewed Whitmer's opus more fully elsewhere on this site, so here I will simply end by saying that "An address to all believers in Christ" is of considerable interest to historians studying Mormonism, since the author (inadvertently) confirms that Joseph Smith was a scryer, using a so-called seer stone placed in a black hat to "translate" the golden plates. Whitmer also reveals that the early Mormon Church had a fluid authority structure, with many of the "brethren" having the gift of prophecy, including Whitmer himself.
While the Whitmerite Church doesn't exist anymore, its members apparently joined a group with similar ideas in 1925, the Church of Christ (Temple Lot), which still exist. This group owns the exact spot where Joseph Smith prophesized that Jesus Christ would land at his second advent!
But, as they say, that's a different story. And, I suppose, a different address...