In A Brief Explanation of The Book of Mormon, it says that the Book of Mormon was compiled from four plates: the Small Plates of Nephi, the Plates of Mormon, the Plates of Ether, and the Plates of Brass. What parts of the Book of Mormon had which plates? The Small Plates of Nephi consisted of Lehi (the lost pages), First Nephi, Second Nephi, Jacob, Enos, Jarom, and Omni. The Plates of Mormon was a abridgement of the Large Plates with Mormon commentating. Those Plates are the bulk of the Book of Mormon. They range from Words of Mormon to Mormon. Between the two there are, Mosiah, Alma, Helaman, Third Nephi, and Fourth Nephi. The Plates of Ether was a record of the Jaredites, and Mormon’s son’s (Moroni) abridged and commentated version is the Book of Ether. Various people read from the Plates of Brass, which was an early version of the Bible, throughout the record, such as during the bulk of Second Nephi, where Nephi reads from Isaiah. There is also the Book of Moroni as the last Book, which was written directly onto the Golden Plates (the plates which contained all of the records in the Book of Mormon).
When you get to Ether you will see that it is actually abridge by MORONI after his father had died. So most of the abridging was done by Mormon, but not Ether or Moroni (obviously).