Soof by Sarah Weeks is the follow up to So B. It (2004). It's written from the perspective of the girl who was conceived near the end of the book. She is now the age Heidi was when she made her cross country journey to learn about her mother.
I must admit that I didn't even realize it was a companion piece when I bought it. It's been fourteen years between books and nearly a decade since I read So B. It. The novel though does include enough explanation in the narrative to fill in the blanks for other either forgetful readers or readers who haven't read the first book.
Aurora has grown up hearing the stories of Heidi and her luck and how she gave her luck up. The story goes that Aurora came to be when luck cured her parents of their infertility. That said, Aurora doesn't believe her parents' stories. Nor does she believe in luck.
This novel involves the visit of Heidi in the weeks before she gives birth to her child. It's a chance for Heidi and Aurora to connect. It's a chance for Aurora to believe a little in the unexpected and unexplained.