Another from at least thirty years (or longer) ago. Set a few years after Earth's defeat and the alien overlords are involved in various power grabs and empire building.
They have been supreme slave masters around the galaxy for countless generations and have evolved until no limbs (everything done by body servants) and therefore use only their tongues for pointing.
I want to say the next slave race down in the hierarchy is semi-evolved to a similar condition as they also have body servers from yet another slave species, quite a few species are in the book.
One of these overlords gets involved with a crime syndicate and is tricked into believing unpainted plastic model planes have great commercial value to humans. He hoards these in glee until he tries to pay off another criminal gang "Why we want that shit?" and realises he's been tricked.
Another alien likes rolling around on human skin rugs and is ever alert for hairy bodied men as it likes the tickle sensation.
I think (!) There is some kind of alien wipe out towards the end (like in Battlefield Earth) and the freed species around the galaxy look to mankind, however I'm not certain on this.
It was a medium size hardback book