Mare's war / Tanita S. Davis.

Enhanced descriptions from Syndetics:

Meet Mare, a grandmother with flair and a fascinating past.

Octavia and Tali are dreading the road trip their parents are forcing them to take with their grandmother over the summer. After all, Mare isn't your typical grandmother. She drives a red sports car, wears stiletto shoes, flippy wigs, and push-up bras, and insists that she's too young to be called Grandma. But somewhere on the road, Octavia and Tali discover there's more to Mare than what you see. She was once a willful teenager who escaped her less-than-perfect life in the deep South and lied about her age to join the African American battalion of the Women's Army Corps during World War II.

Told in alternating chapters, half of which follow Mare through her experiences as a WAC member and half of which follow Mare and her granddaughters on the road in the present day, this novel introduces a larger-than-life character who will stay with readers long after they finish reading.