Hi,
the most comprehensive work is by William A. Dobak: Freedom by the Sword: The U.S. Colored Troops, 1862-1867. It is a quite detailed operational history organised by theatre.
A very readable and also comprehensive overview, but in a more narrative vain, is Noah Andre Trudeau's Like Men of War: Black Troops in the Civil War 1862-1865. It concentrates more on the battles and skirmishes involving African-American troops.
Firebrand of Liberty has already been mentioned - a great book on a very interesting person.
Joseph Glatthaar''s Forged in Battle: The Civil War Alliance of Black Soldiers and White Officers deals with the relationship between soldiers and officers and is a fascinating study. Highly recommened if you are interested in social history.
There are several regimental histories and studies on special subjects, just drop me a PM if you are looking for something specific.
I hope that helps!
Best,
Thomas