When you publish a new version of your book with a significant amount of new content, you can email your readers with a "Release Notes" message.

Most of the time when authors publish new versions, they don't send a message, as they're just correcting typos and such, and definitely don't want to spam readers.

Please note that readers need to opt in to receive these messages, and they can opt out any time, which is another reason not to send too many messages.
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