We believe the Bible to be the word of God as far as it is translated correctly. We also believe the Book of Mormon to be the word of God.
The 8th article of faith outlines the core scriptures of the LDS movement. Even prior to it being written, the Doctrine and Covenants had been introduced into the LDS cannon. The official scriptures of the LDS faith continued to expand with the addition of the Pearl of Great Price, first introduced in 1851 and canonized in 1880.
As the 8th article of faith implies, not all scripture is created equal. By looking at how often various scriptural terms appear in general conference, we can gain a glimpse into the relative importance that Mormon leaders and members place upon various books of scripture in their cannon.
One Mormon prophet declared:
The Book of Mormon is the keystone of our religion—the keystone of our testimony, the keystone of our doctrine, and the keystone in the witness of our Lord and Savior.
While Joseph Smith, the founder of the movement and author/translator of the Book of Mormon primarily cited the Bible during his sermons, usage of the Book of Mormon has increased within the movement over time.
References to the Bible have decreased until about the 1980s at which point they seem to have stabilized. They typically comprise 20-30% of all scriptural references since that time. Belief in additional cannon beyond the Bible and its emphasis appears to be one of the main reasons that mainstream evangelical movements sometimes question whether Mormons are Christians.
Looking at the overall picture tends to obscure some interesting trends with respect to the Doctrine and Covenants and Pearl of Great Price. The D&C in particular is interesting because between the 1890s and 1900s the relative number of reference drops by a factor of 4. The Book of Mormon references increase greatly in 1900 to fill the gap.
In the 1890s polygamy, which had been the defining trait of Mormonism for more than half a century was largely abandoned. During the following decades, missionary work changed and the narrative regarding the first vision was preached widely for the first time. Perhaps Mormonism was seeking a new identity and reinventing itself. Perhaps Mormonism wished to distance themselves from modern revelations (in the form of the D&C) which still sanctioned polygamy. Perhaps there was a new emphasis on the Book of Mormon and the D&C was simply forgotten for a time.
It should be noted that the Doctrine and Covenants was significantly altered in 1921 when the Lectures on Faith (aka the Doctrine part of the book) was removed. Unlike other canonization examples where the church body voted to accept new scripture, there was no vote when this section of the book was removed.
In the 1950s, the D&C made a significant comeback at the same time that the Pearl of Great Price was in decline. The Book of Abraham received significant criticism in 1912 with the publication of a pamphlet claiming that the translations were incorrect. The LDS church used about 1/3 of the space in the Improvement Era, their official publication to dispute these claims for the period of about 6 months. In 1946 Brodie’s publication of No Man Knows My History was also critical about the origin of the Book of Abraham. The most intense criticism however came in the late 1960s when pieces of the original papyri associated with Facsimile 1 were discovered and translated by Egyptologists. These events may partially explain the reduction in reference to the Pearl of Great Price, which now comprises only about 2% of all references to LDS scriptures.
 https://leadinglds.org/what-did-joseph-smith-learn-from-the-book-of-mormon-about-church-leadership-an-interview-with-jerry-smith/ “…Joseph Smith rarely quoted from the Book of Mormon after he translated it. This was the same for Brigham Young and for the general membership of the Church during the early restoration. In fact the Bible was quoted 20 times as much as the Book of Mormon.”