The day commemorates Martin Luther King's birthday (January 15). The holiday was signed into law in 1983 by President Reagan. For years, several states didn't celebrate the day as such for various reasons, and some refer to it by different names. The holiday was first observed in 1986 and was first observed in all 50 states in 2000.
In honor of this day, I've compiled three books about King that might interest you.
Hampton SidesHellhound on His Trail: The Stalking of Martin Luther King, Jr. and the International Hunt for His Assassin
Tavis SmileyDeath of a King: The Real Story of Dr. Martin Luther King Jr.’s Final Year
Doreen Rappaport, Bryan Collier (Ill.)Martin’s Big Words: The Life of Dr. Martin Luther King, Jr.