Did Justin die in the book Mrs Frisby and the Rats of NIMH?
No, he did not. He is alive and well in the sequels, Rasco and the Rats of NIMH, and R-T Margaret and the rats of NIMH.
Mrs.Frisby and the Rats of Nimh is a book wrote by an author named Robert C. O'Brien who lived from 1918 -1973. If you want to know what the story is about, then go to Google and type in "What happened in the story Mrs. Frisby and the Rats of Nimh" or type in "Summary for Mrs. Frisby and the Rats of Nimh