My favorite gospel-enriching book is the Book of Mormon. As I have read and delved into this book over the past 15 years, I have found understanding of God's ways and how he supports his people. Studying the other scriptures in conjunction with the Book of Mormon has also increased my understanding of the gospel.
I love the Book of Mormon. It is an exciting and revelatory book and gives me a feeling of peace that has helped me through some very difficult times. I can really feel the Spirit of God when I read it.