With the free plan, there is a limit of 20 previews or publishes in a month.  The Standard ($8/month) and Pro ($12/month) plans are unlimited.
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We are a small bootstrapped startup, and have to try to make money somehow.
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Finally, there is no offline or local machine mode. However, Pandoc (https://pandoc.org/) does an OK job with making books from Markdown. So, you can use it as an offline preview tool in conjunction with Leanpub.