Most writers know the standard rule to italicize the titles of published works such as newspapers and books. However, what is to be done about the standard works? Each of the books qualifies as what we would term a book, complete with chapters and a multitude of pages (although the Pearl of Great Price is a little shorter than the Bible, of course).
The Church style guide says to "set references to the standard works in roman type, and capitalize them" (6.14). Therefore, we write the Book of Mormon like so. Also, the is not capitalized when writing the Bible or the Book of Mormon, for example. Writers should also note that other sacred works, such as the Koran or the Bhagavad Gita for example, follow the same principles (7.49).
When writing the names of books within the standard works, capitalize the name itself, but lowercase the word book (7.44). As an example, one might write, "I read a few chapters of the book of Mormon in the Book of Mormon yesterday," or, "I love the stories in the book of Ruth."
And just as a sidenote for all of us who like to argue this point: the Church prefers we use "copies of the Book of Mormon" instead of trying to figure out if it's "Books of Mormon" or "Book of Mormons" (6.14).