Not to be mean but I would say that the chances of seeing your paperback books are slim. Many people leave books and it is standard procedure to dispose or cycle those into the crew library.
Your other items "may" be returned but on almost any cruise line, items other than "monetarily valuable" hard items such as jewelry, watches, or documents are rarely returned.
Do your best to make them aware of what you left (which you have done) and keep following up but don't become too optimistic. I also wouldn't put a major red mark on Carnival, the cabin steward or the ship for not keeping or returning it. Most cruise lines could and would easily dispose of these items.
It's a hard thing to lose but one person's treasure is easily another's leftover trash.
It is also your responsibility for removing everything from your cabin before you leave.