Use of Isaiah in 3rd Nephi

Reading 3rd Nephi 21 yesterday and 22 today, I was thinking about the depth of knowledge about Isaiah on display here. In 3rd Nephi 21, the Savior explains the role of the Book of Mormon in the restoration of his covenant people. At the end of the chapter, we read that the covenant people will be gathered in the land of their inheritance. At the beginning of chapter 22, we read "And then shall that which is written come to pass:" followed by a quotation of Isaiah 54. Isaiah in general is hard for me to understand, but reading Avraham Gileadi's commentary really helped: http://www.isaiahexplained.com/54#commentary. This also helped me realize how well it fits into the context of the preceding chapter and that the author must have had a really good understanding of Isaiah. We see the same in chapter 20. There are quotes from Isaiah and Micah dispersed in between the words of the Savior when he gives his sermon to the Nephites. I find The Book of Mormon is really a gateway to understanding the Old Testament better.