The Girls Leadership Book Club is, in my opinion, the go-to resource for finding empowering, age-appropriate reads for your girls. I highly recommend you join their Facebook group to get their fall 2018-2019 school year recommendations when they come out.
This list is from their recommendations for the last school year by age group. My daughter has read several on their middle school list and loved them. I’m sure you will too.
Who knows, maybe you’ll be inspired to form a book club of your own!