This was linked to a few months back when the site was just a teaser. The book officially launched today and is available in various eBook packages. Here's a summary from the author "Sketch App Essentials is the perfect guide to help you realise the full potential of this amazing application. Learn Sketch in the shortest possible time, with the most practical, and informative guide there is."

Hi, I'm Marc, the author of Sketch App Essentials. With this guide I wanted to focus on creating 'real-world' applications, make the guide as practical as possible, and to get you up to speed in the shortest possible time. Cut down on the fluff, and just get you in there ‘doing’. Make it easy for beginners to dive straight in and feel comfortable, and informative enough for long-time users of Sketch to find it of value. Hope you enjoy it!