I renamed my chapters with Retitle and I prefer the old titles. How do I get them back?

Right after renaming, press "Undo" in the Retitle window. Later, from version 1.9.24, open the book's page and press "Retitle" again: a line says how many chapters were renamed and on which date, with a "Bring back the old titles" button. Click it and the titles from before the last Retitle come back. A chapter you renamed by hand since then keeps the title you gave it.