The book "Bud, Not Buddy" by Christopher Paul Curtis is not in the public domain. Published in 1999, the book won the Newberry Medal in 2000 and has been in publication since then.
There is no indication that the author has explicitly relinquished his copyright to "Bud, Not Buddy," so the 1999 title is copyright protected. By law, this protection continues during the author's life plus 70 years. "Bud, Not Buddy" also won the Coretta Scott King Award. Christopher Paul Curtis published a number of other successful children's titles, including another Newberry Medal winner, "The Watsons Go To Birmingham - 1963".