My daughter and I are big fans of Brad Metzler's "Ordinary People Change the World" books. We already own his books featuring Rosa Parks and Jackie Robinson. What we love are the wonderful illustrations by Christopher Eliopoulos and the deft balance of potency and humor from Brad Metzler. I am Martin Luther King, Jr. is a wonderful tribute to Dr. King and contains some powerful advice for children and the adults in their lives. ("You must never feel you are less than anyone else." "There is power in words.") I also appreciate how each person who is featured in this series is shown to be human and far from perfect. If you are looking for a book to discuss the legacy and importance of Dr. Martin Luther King, Jr., check out this book, which is also available in the iTunes bookstore.
Further reading: here's a link to a review I wrote last year of another good book, Martin and Mahalia, that focuses on Dr. King.