A common quote once observed that trying to pin down Mormon doctrine is like trying to nail Jell-o to a wall. Even for people raised in the religion, determining what constitutes Mormon doctrine and what does not can be confusing. Even though there is a book on the subject called Mormon Doctrine by former apostle, Bruce R. McConkie, it is not Mormon doctrine, but in the words of a still-living apostle, “well-considered opinion.”
Given that a definitive book on Mormon doctrine cannot be used as a guide, someone studying the church's theology u needs to know what sources are official Mormon belief. This is not easy. A prophet can make a declaration that can be belief as can many in the Quorum of the twelve, but it may just be their well-considered opinion. The most reliable source for the Mormon doctrine is the four standard works.
The scriptures of the Mormon Church are the sources of what Mormons believe cannot be contradicted. Although like many other religious texts, there are passages that contradict each other., such as the nature of the Trinity described in the Book of Mormon and the story described in the First Vision story,, a person studying Mormon doctrine should assume that the latter version is correct Mormon doctrine.
A closed canon, such as the one used by Catholics, would stop a lot of the confusion about what is Mormon doctrine. In many cases, according to the Christian Resource Institute, a site biased against Mormons, the only thing that Mormons must accept as official doctrine is the scripture.
What Mormons believe may vary widely; some accept every word from the current prophet and many words of past prophets as scripture while, as mentioned above, some maintain that Mormons only have to accept the standard works as doctrine. A hybrid approach is employed by many Mormons where they accept what they feel to be true.
This approach is rooted in Moroni’s Promise, which can be found in the last book of the Book of Mormon. After reading the book for the first time, the reader is asked to pray about the book and receive a revelation of whether it is true or not.
While the tenth prophet of the Mormon Church, Joseph Fielding Smith, felt that new Mormon doctrine should not coincide with previous Mormon doctrine, many of the revelations of Brigham Young,such as blood atonement or the Adam-God doctrine, are no longer considered to be part of the canon of the church.
The straightest possible answer to this question is that Mormon Doctrine is a book by Bruce R. McConkie. Individual Mormons hold their own opinions about what teachings of the church are official doctrine and which teachings are merely well-considered opinion.