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.

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

Or

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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.

Or

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

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

E) Take a solid year to learn how to use Amazon or Facebook ads and be prepared to lose quite a bit of money as you figure it out.

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

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

A) that are not actually books.

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You cannot effectively promote books

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) that are unreadably terrible in any other way.

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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.