I assumed she was male, as, at the time, I only know 'Andre' as a man's name (I'm British, and it's rare here. The only Andre I knew of was the [male] conductor Andre Previn.)

It's generally a man's name in the US, as well. Andre Alice Norton was originally named Alice Mary Norton; she used Andre Norton as her pen name in an effort to widen her audience, and then later adopted it as her legal name.