The version that avoids plagiarism by using proper MLA citation style is:
Alexie realized he needed to leave the reservation when he opened one of his high school texts and saw his mother's name written inside - the school was using thirty-year-old books (Alexie 26-27).
This version includes the author's name in parentheses and a page range indicating the source of the information, and is consistent with the MLA citation style for in-text citations.