If you go to https://leanpub.com/YOURBOOK/edit_generation_settings and scroll to the bottom, you'll see the following:

DISPLAY YOUR BOOK AS A WEB PAGE ON LEANPUB?

No web page
Just the sample
The full book

This feature lets you generate a web page of your entire book (or just the sample) and put a link to it on your book's landing page.

To turn it off, just choose "No web page".

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