Do you have a summer tradition? A place you go on vacation every year? A special spot for ice cream after dinner? A friend whom you only get to see when the days are warm, extra long and marked by endless hours of little to do?

Sara Pennypacker’s book about two girls spending summer with a relative on Massachusetts’ Cape Cod is about those types of traditions.

It just takes a while for the girls, Stella and Angel, to realize how much they have in common and how much they can share over the summer months.

Stella loves being at her Great Aunt Louise’s house. Louise represents all the stability, routine and family structure that Stella’s unpredictable mom can’t offer. But 11-year-old Stella is less than thrilled about Angel, the foster daughter that Louise has brought in to her house.

There’s a big event that happens early in the book (we can’t tell you or it would spoil the surprise) that will decide whether Angel and Stella will be friends or enemies. There’s no doubt that this is a summer that will change the girls forever.

That’s just what author Pennypacker was going for in writing her novel. She said that she had a special summer friend named Sonja on which she could model the relationship of Angel and Stella. “In the summers we’d hang out every single day, then on Labor Day we’d each go back to our real homes. The next summer, there we’d be, every single day as if the school year had never happened.”

Pennypacker, who spent most of the summers of her life on Cape Cod, told us that she sees summer as a book. “To me summer is fiction; the rest of the seasons are all non-fiction. But best of all, I think, is that in summer kids can be outside. On Cape Cod, night-swimming was always magical to me — that’s why I wrote a scene about it in “Summer of the Gypsy Moths.”