A long time ago, GitHub deprecated their Services service, in favour of either (a) GitHub Apps or (b) Webhooks.
We actually built a GitHub App and a Webhook approach, to test both.
Even though the GitHub App was fancier, we decided to use the Webhook approach, for the following reason:
It is more explicit about what it does. We don't want access to your entire GitHub account. Yes, we could ask for the appropriately-fine-grained permissions, but then you'd have to read a dialog and trust GitHub or us. The Webhook approach seems a bit clunkier, but it is completely transparent about what is happening.
Here is a page with links to the instructions for setting up Webhooks, depending on your Leanpub writing mode:
With this, we are officially now making the Leanpub GitHub Service is now unsupported. It may or may not work, and we will not spend any time supporting it. And once GitHub completely kills Services (I think this is in January 2019), then it definitely won't work!