The Book of Mormon is not the primary source of Mormon doctrine, contrary to popular belief (and contrary to the missionaries’ practice of exclusively promoting that book to new/potential converts). It is primarily presented as a historical narrative written by people who there is no secular record of. These accounts were purported to have originally been written in a language unique to the BoM on golden tablets which were given to Joseph Smith by an angel and were returned to that angel after Smith’s translating of them. So the original document is not accessible nor is the original author.
Whereas Doctrine + Covenants and Pearl of Great Price are openly authored, purportedly via divine revelation, mostly by the founder of Mormonism, Joseph Smith. These writings are primarily instructional and didactic (meant for teaching). So not only is the genre of writing directly intending to set out a foundational theology for the religion, the written documents and the author are directly accessible via secular history.
Interestingly, the first portion of the D+C was removed by the church in 1921. These were seven lectures by Smith setting forth basic church doctrine. These are now called the Lectures on Faith.
Comment below your thoughts on the Mormon scriptures! Are they trustworthy sources of truth? What in them distinguishes Mormon doctrine from other religions?
[Pictured above is a statue of the angel Moroni looking out at the Mormon temple in Salt Lake City, UT]