When my mother was here after Elizabeth was born, she was reading a book called Franklin and Lucy, which was about FDR and his longtime mistress Lucy Rutherford. She said it also talked abut the other women in his life, including his wife. It looked interesting, so I tried to check it out at the library. I ended up having to reserve it - I just got an e-mail telling me it's ready for me.
In the meantime, I checked out FDR and Lucy, which is also about FDR and Lucy Rutherford. FDR met Lucy when she came to work for Eleanor Roosevelt as her social secretary. When Eleanor started to suspect that Lucy and FDR were having an affair, she let her go, but FDR promptly hired her on at the Navy Department, where he was assistant secretary at the time.
Eleanor offered to give FDR a divorce so that he could be with Lucy, but Sarah Delano, FDR's formidable mother, threatened to cut off her financial support. Instead of a divorce, FDR had to promise never to have contact with Lucy again.
Turns out he didn't keep that promise, and his daughter and secretaries enabled his meetings with Lucy through the years. Lucy was even with FDR when he died; she was whisked away before Eleanor arrived and her presence was kept out of the press for decades.
I'm still going to read Frankling and Lucy. From what my mother said, it goes into more detail about more people in FDR's life, including Eleanor.