This is my first post on this year's Book of Mormon curriculum. LDS.org posts the course manual. Lesson 1 includes material on the keystone metaphor, as well as handy links to most of the scriptures referred to in the lesson. I'll try to post once a week on a topic related to the lesson for the week. The topic of this post is the keystone metaphor, based on a statement by Joseph Smith: "The Book of Mormon [is] the most correct of any book on earth, and the keystone of our religion" (HC 4:461). My main point is this: It's just a metaphor!
What is the Book of Mormon and what is its place in the modern Church? First, it is a book, not a keystone. It was published by Joseph Smith and his small but loyal group of supporters in New York in 1830, shortly before the LDS Church was first organized. It is part of the scriptural canon of the LDS Church. Along with the Bible and unlike the other canonized LDS texts, it is highlighted in the Articles of Faith: "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." It gets one full year in the four-year rotation used in Sunday School and LDS Seminary classes, and passages from the Book of Mormon are often given to Sacrament Meeting speakers to use as the basis for talks to the congregation.
Members of the Church are encouraged to read the Book of Mormon regularly. When missionaries teach those who are interested in learning about the LDS Church, they invariably give that person a copy of the Book of Mormon, ask them to read selections from it, and invite them to pray about it and gain a spiritual confirmation. All of these things illustrate the central role the Book of Mormon, along with other scriptural texts and items of faith and belief, occupies in the LDS Church.
But I think overemphasis on the keystone metaphor can be misleading. Note that AOF 8 lists the Bible first; the claim about the Book of Mormon is relegated to an "also" clause. The Bible is as central to the doctrine and practice of the LDS Church as the Book of Mormon, and the keystone image, which is just a metaphor, shouldn't displace the canonized Articles of Faith statement, which gives at least equal weight and arguably more weight to the Bible. Furthermore, as I recall, the Book of Mormon is not the subject of any temple recommend interview inquiries, whereas several other items of faith and practice are, which likewise suggests the keystone metaphor overstates the formal role of the Book of Mormon in the Church.
So use the keystone metaphor if you must, but don't get carried away with it. LDS.org provides a correlated and hyperlinked summary of the Book of Mormon, along with a list of articles by LDS leaders and other resources. Check out the short video of Elder Ballard contrasting the scientific versus the spiritual approach to the Book of Mormon (you need to hit the start button for it to play).