Nikki Giovanni is perhaps best known for forging the use of the lowercase 'i' as a first person singular pronoun, later used by writers such as bell hooks, though really her legacy should be much more than that. Giovanni is one of the most accessible yet still palatable poets I know. Her work is rarely obscure or explicitly erudite, but her themes go beyond greeting card verse.
Giovanni's work spans over a dozen volumes of poetry, though my personal favorite is My House; her fourth book of poetry, it was published in 1972.
