Yes, and you can start working with it today. You can use it to integrate everything in Pancake from a third-party app.
Documentation is still very much an ongoing effort, but you can find some details here.
We also have a PHP SDK, if you're working on integrating Pancake with a PHP-based application. You can access it at: https://github.com/pancakeapp/php-sdk
The API SDK is still a work in progress but we're committed to improving it as quickly as possible. To that end, all pull requests or issues raised will be treated with the utmost urgency and will be resolved and merged in 48 hours.
We're working on improving both the API and SDK as much as possible, so if you see have any questions or see anything missing, or that you need help with, feel free to let us know by starting a free support ticket or starting a live chat. We know the entire Pancake codebase off by heart and are likely to be able to immediately answer any query you might have. Additionally, we have the ability to make modifications to the core, so if you need anything that's missing, it's easy for us to add it for you.