I deleted a chapter by mistake. Can I get it back?

Yes, in version 1.9.18 and later. On the book's page, above the manuscript list, a button "Deleted (n)" appears as soon as the book has a deleted chapter. Click it, find the chapter (its title, word count and the date it was deleted are shown) and press Restore. The chapter goes back to the place it had, or to the end of the book if that place no longer exists, and the chapters after it are renumbered again. Its earlier versions come back with it.