I hit the 20 preview limit. Can I make previews offline or have the limit increased?

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.

We are a small bootstrapped startup, and have to try to make money somehow.

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.