For book narration, I am female, what are some characteristics I can add to my voice to sound more manly besides just lowering my voice?
There’s not an easy answer to this because it depends on the character. Is it a man who is quiet and measured, or brusque and terse? Perhaps a man could be less musical in quality and tone, but you have to be careful about imposing qualities with the goal of sounding masculine that would risk missing the character. I would concentrate on allowing the character to inform the voice. Then you will know how to articulate, where to place the resonance and so on. If you are clear about who is speaking it will be less about affectation and more about getting the meaning across. Then the listener will be clear as well.