You asked about appropriate books. Two books for elementary school children with ADHD came out in 2016: Baxter Turns Down His Buzz by James M. Foley for ages 4-8 and What Were You Thinking by Bryan Smith. It is worth checking these out! Maybe your local library could order them so they could be used by other children too.
It sounds like you have thought about this and have been trying different things for a while. Social skills are difficult to learn as peers reject others without giving feedback. What did her teacher say, regarding the exact reasons why other children have rejected her? Which social skills does her teacher recommend working on over the summer? Is she monopolizing conversations or the teacher’s time? Is she interrupting others or saying things that do not follow the conversation? Does she focus on herself and avoid listening to others? Is her energy level much higher than her peers? Does she wear-out emotionally and act-out when others are calm and engaged in the activity? These are just a few out of many specific skills she might need to learn.