How do I rename a part, add a short introduction to it, or remove it?

From version 1.9.24, hover the part's row on the book's page. The pencil icon ("Rename this part") opens the part's title and a box for "A few lines for the part's page (optional)": change them and press Save. Those lines are printed on the part's own page. The trash icon deletes the part only: its chapters stay in the book, with their text and numbers, and join the part before it. In a translated edition the part rows cannot be changed, because the edition follows the original book.