>>24453170 (OP)I'm going to give a real answer for the one serious person who wants to advertise their book. I run a marketing agency (not for books though).
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Your starting point should be main stream social media - it is currently the most effective way of reaching a targeted audience at scale. The algorithm is designed to connect your content with people who are interested in it. If your book has a somewhat mass market appeal (Within the demographic of readers) then short form content will be the best. However, if your target is quite niche, longer form content will be better.
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In terms of what to post exactly, its a three step process.
First, identify who your target audience is. Get as specific as possible about exactly what kind of person would be interested in your book.
Then, identify what kind of content that type of person consumes, what platforms, etc.
Finally, post that type of content on those platforms to attract those kinds of people.
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Then post so much volume of high quality content that it would be unreasonable for you not to succeed. This entire strategy is free, but it requires work so you won't do it, you'll just complain. Did you think marketing something was easy? Best of luck.