How does a new KDP writer supposed to market a book?

Because you cannot persuade readers to buy any book in which sentences do not make sense. This shouldn't be “does.” If you don't see the problem, that is a big problem.

A) Build a following on social media, a following of people who will buy your books. Good luck with that. There is very little evidence that social media can translate to sales.

C) Persuade book reviewers on BookTok or Substack or someplace to recommend your book. If your book is unreadably terrible, this is probably impossible.

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D) Pay promotion services like Written Word Media to promote your book. Legitimate services will not accept your book for promotion if the presentation is crappy or the book is badly written.

B) Build a newsletter mailing list of people who will buy your book because they trust you to write books they want to read. This is slow, but ideal.

C) that are unreadably terrible in any other way.

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D) that are presented with bad covers, bad descriptions, or ludicrously bad interior formatting.

You cannot effectively promote books

Are you planning to proofread your book to avoid sentences such as

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How does a new KDP writer supposed to market a book?

B) that are filled to the brim with typos or errors.

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If your book is well presented, well written, and basically free of typos, then

A) that are not actually books.