An encyclopedic review of most of the major doctrines in LDS theology. He follows the progression of thought from the ancient Hebrews, through the early Christian era, early 19th century American Christianity, and early Mormonism into the modern understanding. It was really fascinating to see how much somethings changed even in a short amount of time. Mormonism started out thinking, looking, and behaving like any other protestant 19th century American religion. The early revelations and the Book of Mormon are not notable for their doctrinal differences. Slowly, things evolved.
Take the nature of the Godhead. Before the early 1940's it was repeatedly taught that God was a spirit not corrupted by the base trappings of material. The Lectures on Faith describe the Father as "a personage of spirit." Parley P. Pratt wrote in 1840, "Whoever reads our books, or hears us preach, knows that we believe in the Father, Son, and Holy Ghost as one God. That the Son has flesh and bones, and the Father is a spirit...a personage of Spirit is in the likeness of the temporal body, although not composed of such gross materials as flesh and bones." It was not until a year later do we have any records of Joseph teaching that the Father has a physical body.
The Holy Ghost was not originally considered a member of the Godhead. The Lectures on Faith states the Godhead is comprised of "two personages." The Book of Mormon and the early revelations in D&C refer to the Holy Ghost as "it" rather than "he".
Doctrines progress in a non-linear fashion. It isn't that we are learning new things unknown before, but that we learn new things that contradict previous discoveries. This can be seen in contradictions between Joseph's early 1830's translation of the Bible, and his teachings a decade later in Nauvoo. Revelation 1:6 reads "God and his Father". The Joseph Smith Translation changed it to "God, his Father." In 1844 Joseph declared the verse "altogether correct in the translation," indicating it to mean the Father of The Father.
This book covers everything: apostasy, restoration, priesthood, Satan, Godhead, preexistence, fall, atonement, gospel plan, Zion millennium, and resurrection. I learned a tremendous amount about Hebrew beliefs. Often, we impose our present understandings onto the past, rather than let the Old Testament speak for itself. For example, there is very little after life in the Old Testament. All prophecies and promises were generally temporal in nature. In fact, Ecclesiastes 9 explicitly states that this life is all there is.
One criticism is that Harrell covers too many topics, and doesn't treat with enough depth the complex and nuance history. Each topic, alone, would deserve a book-length treatment. Nevertheless, it does excel at its main thesis: that there are varying and contradicting doctrinal voices in all revealed scripture and prophetic teachings.
Theology is a meeting between man and God and takes work on our mortal part. The nature of the Godhead, in Mormonism, remained unsettled until the scholar, Elder Talmage, worked out the doctrine in his Jesus the Christ. His solution worked. The first presidency and the quorum of the twelve formalized his take in an official statement the year after JtC was published.