on Jane Friedman site:

Of all the tools available to authors to promote their books, advertising may be the most challenging.

It seems so appealing: set up a campaign and watch the money roll in. It rarely works like that. Advertising is tricky. Many authors have blown their budgets on print or online ads with little to show for it.

While advertising helps you reach people beyond your immediate sphere of influence, the key is to use it strategically. Advertising should support your book marketing strategy, not replace it.

Many authors use Facebook and Amazon to advertise their books. If you’ve tried these platforms without success or hesitate to spend the money, consider experimenting with BookBub ads.