President Monson said, "If you have not read the Book of Mormon, read it." Then he made this promise that I know and have a testimony of being true.
I was a member for several years before having received a true and strong testimony of the Book of Mormon. I always knew it was true, and I had faith in it, but I never really received a true witness for years after my baptism. After having received my personal witness, I learned of the strength the Book of Mormon offers to those who live according to the words of the prophets who testify of Christ within its pages. It is a powerful source and it has greatly blessed my life.
It took me several years to realize I had the most important and effective "instruction book" at my disposal.