In terms of help with webhooks and API keys: our Free plan has a limit of 10 previews or publishes per month.
Chances are you'll bash into the 10 preview or publish per month limit pretty fast if you're on the Free plan and using webhooks to automatically preview or publish whenever you commit and push.
Now, if you're at this point, you're probably getting as much value from Leanpub in a month as you are from, say, Netflix, and hopefully we're worth a subscription. Even the Standard plan has unlimited previews and publishes.
We're a bootstrapped startup, funded entirely by what we earn from subscriptions and the Leanpub store. We really appreciate every subscription.