What is Calpurnia's surname?
While Calpurnia's surname is never mentioned in the book, this seems to mesh with what Scout knows about Calpurnia - just enough.
For example, when Calpurnia takes Scout and Jem to her church, First Purchase, the children get a peek into Calpurnia's world and life rather than simply seeing her as a player in their own. Scout notices when Cal replies to Lula that "I thought her voice strange: she was talking like the rest of them" (Ch.12). In fact, these are Calpurnia's people - her friends, family, neighbors, and so on. They are not "them" to her - they are "us." But in Scout's world, Calpurnia speaks and acts differently.
Because Calpurnia is "the help" it is entirely possible that Scout simply does not know her last name. Although Atticus treats Cal like an equal in some ways, she is still their servant. We understand this from many scenes, but as one example, when Calpurnia has to stay overnight at the Finch home, she sleeps on a cot in the kitchen. Certainly, white guests like Aunt Alexandra would not be expected to do so.
Not knowing Calpurnia's last name re-enforces to the reader that she is not quite equal in all eyes. We know the last names of even minor white characters, but not the surname of one of the most respected African American characters.
I've always wondered about this myself, but only author Harper Lee knows for sure. Calpurnia's surname is never mentioned in To Kill a Mockingbird, perhaps because Scout never bothered to ask Cal--or Atticus--what it was. We do know that Calpurnia is a grandmother, but we also never find out if she is married or has a husband or if he is still living. Her son, Zeebo, is mentioned several times in the story. Zeebo drives a garbage truck, and he disposed of the body of the rabid Tim Johnson after Atticus shot the dog. Zeebo also attends Cal's church, and Scout mentions that Zeebo has some children himself who are nearly grown.