Meet Jamie Sumner. She taught high school English for more than a decade before starting to write books for middle-grade readers — generally between 8 and 12 years old. The main character in her first book, “Roll With It,” has cerebral palsy and is in a wheelchair. Her book “Tune it Out” features a character with a sensory processing disorder. We spoke with her about her book , “The Summer of June,” which takes on the issue of anxiety.