Mormon writes this line. Because the chapter ends, it is not obvious that Mormon will continue his narration for many verses to come. At the end of Amulek’s discourse, Mormon had written: “And now the words of Amulek are not all written, nevertheless a part of his words are written in this book” (Alma 9:34). We see a very similar transition here. In Alma 9, those words did precede a chapter break, but they do not here.
In both cases, Mormon is noting that there are two kinds of “books” he is working with. The first is his source, and in both cases, he indicates that his source has more information. That source might be the large plates of Nephi, or it might be a separate record, such as the record of Alma that he is using for this part of his narrative. The second is the “book” he is writing, or the plates upon which the Book of Mormon was written. When he says that things are not written in “this book,” he means the plates of Mormon upon which he is writing.