On the Leanpub landing page for a book that has not been published yet, at the top right of the page, you will see an option to sign up to be notified when the book is first published.
Here's an example of what that looks like:
Using this form, you can enter your name and email address to sign up to be notified by Leanpub when the book becomes available.
You will also see an option to "Also share your email address with the author". If you tick this box, your email address will be shared with the author, and they may contact you or e.g. add you to their author newsletter, if they have one. (They're not allowed to use this information to spam you, or anything like that.)
Finally, using the "I WOULD PAY..." feature, you can indicate to the author (with no obligation, of course!) how much you think you'd be willing to pay for the book when it's published.
Once you've filled out the form, click the "Notify Me When This Is Published" button.
OK, that's it!
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Interested in creating your first Leanpub book? Here are some quick tutorials for our most popular writing modes: http://help.leanpub.com/en/articles/3088382-quick-walkthroughs-for-getting-started-on-a-leanpub-book