One of the strengths of the Fate Core is its ability to let the players and the GM create pretty much any world they can imagine. But years of playing in one fantasy setting or another can make it hard to break out of that mold decisively. The Secret of Cats let's you do that easily. But more than that, it is a fun and evocative setting that is much darker than you'd think.
Throw in some amazing art and it is well worth the $4 suggested price.