The biggest spenders in the marketing business are doing so primarily to justify their own size. That is to say, if you're a trillion dollar business, you are going to spend a billion dollars on advertising purely to say that you spent a billion dollars on advertising. The quality of the data - or the clicks - matters less than the fact that you've bought your brand a sense of ubiquity. It's fractally nested corporate classism.

Direct response marketing - i.e. when you are buying ads specifically to get someone to buy a book - is a far smaller fraction of the market. It had a moment in the early 2000s when Google and later Facebook figured out how to extract lots of information about their customers and sell it on to other direct response marketers. But even then, the really lucrative marketers aren't legitimate businesses, they're scammers using the ability to micro-target ads to find their biggest rubes as cheaply and silently as possible.

If you're a regular person trying to buy ads for a legitimate business, you're probably going to get swamped by all of this and taken for multiple rides by several different kinds of scam.