In Another Life, We Were Braver

It started anonymously-letters carved into wood, then written and legitimized on paper.

They made 4 rules:

Use plain paper and ink.
Don't use any identifying details, names, or adjectives.
Never EVER reveal your identity.
You tell my secrets, I tell yours.

These rules became Marlene's lifeline. The only time she could tell her whole truth to someone without fear of judgment. It would be two years before she understood the identity of the other girl, fearful of vulnerability for all those years.

Or

Theodora Pettigrew had always done things properly-until Marlene McKinnon came into her life and made her realize she might have been doing it wrong all along. When she changes, it may be too late. The war looms overhead, and every wrong decision could cost them everything.