We know a great deal about the 54th Massachusetts because the commander of the regiment, Robert Gould Shaw, wrote many letters telling of his experiences. To whom did he write these letters?

to his family and  friends.

Explanation:

Born on 10 October 1837, Robert Gould Shaw served as an American officer in the Union Army until his death on 18 July 1863. During the Civil War Shaw had written more than two hundred letters to his family and friends. These are housed at Harvard University's Houghton Library. He was popularly portrayed as a martyr.  His mother edited an early collection of his letters and sought to maintain that her son perished for the abolitionist cause.