Nephi made a large set of plates, upon which he engraved his father’s record, which is often called the "Book of Lehi." What happened to the Book of Lehi? Why does the Book of Mormon not start with Lehi chapter 1? Well, the translation of contents from the Book of Lehi was lost with the 116 pages that Martin Harris borrowed to show to his wife and family. Will we ever have the Book of Lehi? Yes, some day. Will we have the brass plates? Yes. We will eventually have all of the important records of the Nephites, including the two-thirds portion of the Book of Mormon that was sealed.
Book of Mormon Central, "How Does the ‘Mosiah-First’ Translation Sequence Strengthen Faith? (Words of Mormon 1:5)," KnoWhy 503 (February 22, 2019).