question-  if I post a wl book to a friend and that friend doesn't want it, how do I cancel so it doesn't go fifo?

You can't.  If they decline, it automatically goes FIFO.  I'd send a PM before you post and ask if they want it.

Post the book and then put your Bookshelf on Vacation Hold. The book would get "trapped" on your Bookshelf and you could delete it if they declined and then you could remove the V Hold. Stops them from seeing your Bookshelf and ordering more though.