Hello MrX,

The setting for Folders above Bookmarks vs. Bookmarks above folders vs. Mixed is at the bottom of Settings ▸ Sorting as in your screenshot.  This setting is applied to all folders in your document.  This cannot be set individually for each folder; hence it does not appear in the Inspector.

Note that all of the settings in that tab are applied when sorting is done and don't happen instantly.  In your screenshot, you are set to Folders above bookmarks, so that you'll get what you want after you click in the main menu: Bookmarkshelf ▸ Sort All.  If this is not your usual preference, you can set it to one of the other two and Sort All again when you are done.

This will only work if the folder of interest is set to will be sorted.  In this Help Book section, look at the first two icons under Meanings of Icons in Content to make sure the pertinent folder will be sorted.  If not, use the Sort this folder? popup in the Inspector, or keyboard shortcut ⌘7 to toggle it.

Finally, before you spend too much time moving bookmarks around, know the tips given in this 45-second screencast.

I try to keep my bookmarks sorted in folders and subfolders. Unfortunately, I also have a lot of "loose" bookmarks that haven't made their way into folders yet...

At some point I sorted things and wound up with individual bookmarks mixed-in with the folders. I would like to place those loose bookmarks below the sorted folders so it is easier to move or delete them.

I currently have my sorting preferences as shown in the attached files but I don't know how to apply this to specific folders so everything remains mixed-up. I suspect that I am missing an important setting somewhere...