I've been reading 'Trans', by Helen Joyce, and just happened to be on a city tram, and pulled it out, not thinking anything of it.
After a few minutes, I noticed a youngish edgy looking man with a skateboard peering at the book cover. I don't know what he was thinking - maybe 'This is a hateful Terf', or 'This looks interesting'
But anyway, I think publicly reading GC books like 'Material Girls' or 'Trans', is a good way to encourage others, without direct communication.
If someone does approach, you can always say - "I'm just trying to learn more...have you read it?"