To answer your question: Try searching the Kindle store for something that is in Kindle Unlimited. For example: http://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias%3D... You'll see "$0.00 kindleunlimited" for books that are available. The selection is grim.
I searched for a philosopher I like to read and none of his books had the "unlimited" indicator... so I don't think this is for me.