To add to the above answer, a literal comparison between I Know Why the Caged Bird Sings and The Color Purple is the exploration of sexual violence that both books address.  In the first text, eight-year-old Maya is raped by Mr. Freeman, her mother's boyfriend.  The horrifying experience causes Maya to become reclusive, and she shuts everyone out of her world except her brother, Bailey.  Maya is robbed of her identity and feels like she has no place in the world.  Similarly, in The Color Purple, Celie is raped by her father and is forced to bear his children.  Celie endures repeated sexual abuse by her father until she is sent into a marriage with Albert.  Celie also feels like her life means little, and she retreats into herself, expressing her thoughts only through letters to God, and later to her sister, Nettie.  Maya and Celie eventually find their inner beauty through close relationships with other women, Ms. Flowers for Maya, and Shug Avery for Celie.

There are many similarities between these two books, but the easiest one to address is the position of black women in society, because they had no real rights at the time.  Both books are about women who were stripped of their identity by overpowering societal elements and forced to swallow their pride and their voice.  What makes both books so vital is the ultimate sense of empowerment felt by the main characters once they take hold of their own lives.  Although it takes these women a long time to discover their inner power (and beauty), they ultimately find their value as human beings - even in the face of physical, mental, and emotional abuse.  This is a lesson that transcends race and gender, and becomes inspirational to all people in any historical time period.