Edit book: Save the state of the current book when opening a new book…

… via File->Open book

src/calibre/gui2/tweak_book/boss.py

@@ -350,6 +350,8 @@ def open_book(self, path=None, edit_file=None, clear_notify_data=True, open_fold
         if self.file_was_readonly:
             warning_dialog(self.gui, _('Read-only file'), _(
                 'The file {} is read-only. Saving changes to it will either fail or cause its permissions to be reset.').format(path), show=True)
+        with self.editor_cache:
+            self.save_book_edit_state()
 
         for name in tuple(editors):
             self.close_editor(name)