New paragraph indentation is a "global" setting that is included by default in the Fiction book theme:
If you switch to the Business or Technical theme, by default new paragraphs will not be indented:
You can also change this setting if you have a Standard or Pro plan, and you select the "Custom" theme.
You can find the Book Theme page by going to the Overview page for your book and selecting "Book Theme" under "Settings".
The Overview page for your book can be found here: