In my twenties (in the 90's,) I bought a lot of books at a used book store and would frequently donate them to local libraries or sell them back.
There was one paperback that has come to mind recently when the wife started volunteering at the local humane society.
It features a kind of dumb human and a sentient cat. It's set in modern day USA. Some sort of crisis comes up and the cat reveals himself to be an alien. I don't think all the cats on earth are sentient, but some are and they are here studying us. Little things I can remember is that the cat is pretty snarky, the human isn't too bright, and the "villain" is also a cat, i think. It's pretty light hearted as I recall.
It's driving me a little nuts, because I remember bits and pieces but I can't even recall the title or even cover art. It may have been one of the many 'stripped' paperbacks that get thrown away without the front covers.