What have we learned about Clymnestra's relationship with Agamemnon in the first six books?

Clytemnestra and Agamemnon's marriage was political in nature, a marriage without love. Thus, when he returned from war she helped his cousin to murder him. Her own son Orestes then killed her to take revenge for his father’s death. Clytemnestra was the sister of Helen, but by a mortal father, meaning that she is not a daughter of Zeus.