On Slack by Standuply - The most extensive book on using Slack to the utmost
Alcamy - Free, open self-learning platform. Learn & teach anything.
Startup Playbook by Hatch Quarter - A guide to building your startup
Stacks - A bookshelf for your online courses
Marketing for Startups - 47 practical tactics to get customers and achieve hockey stick growth.
Airschool - Airschool is a live-streaming educational platform with engagement and monetization tools to help anyone start teaching online. It can be finance, business, wellness, meditation, branding, marketing; in short, anything you want!