Up to this point, Mormon has been abridging the large plates of Nephi, with additional material taken from a few select separate sources, most notably Alma’s personal record and Nephi’s (son of Nephi) personal record.
Now, Mormon leaves behind his role as either official record keeper or abridger of records. This is his own record, viewing his own life in retrospect. While many of these events would have been recorded on the large plates that he also kept, this is a record written particularly for this project.
Nevertheless, when he says that he calls it “the Book of Mormon,” it would be more appropriate to keep book in lowercase, as “the book of Mormon.” This “record of the things which I have both seen and heard” is his personal book, not the larger work to which it is attached. This is his intended closing to his masterwork.