Ok, so I know that you can share specific books and hide specific books, but obviously you would hide the adventure you are runing. How does it work with Items that are in that adventure, or character options? Is there a way I can give that specific stuff to my players, but not the rest?

You cannot actually “hide” any of the books. All you do is block their access from reading the compendium, but everything else still gets shared. So if you block access to the advertiser, they won’t be able to read the book at all (unless they have access somewhere else), but all of the Magic items etc. stay available for them to add to their sheets.