Apparently Amazon changed the behavior of the "Books" folder in a recent Fire OS update with the result that books CC stores in the "/Books" folder no longer appear in the device library or on the "recent" list. The "fix" is to tell CC to put the books in a different folder.
See the MobileRead thread Kindle Fire: Books no longer showing in library or recents list for a discussion and investigation of the problem. "Experiment 3" in post #8 presents one workaround. The last post (#9) describes another way using an SD card.
CC starting in V188.8.131.52, the default storage folder in /Kindle instead of /Books. This won't change an existing installation. Instead you must change it in CC using Settings / Formats and folders.