Book excerpt: Obsidian (at obsidian.md on the web) is a new, free app for helping you build a "second brain" - a place for your ideas to arrive, develop, and stay for the long haul. The Obsidian app is built on open standards that ensures that your second brain will always be your own. It works with familiar files and folders that stay local on your disk, ensuring privacy and longevity. Yes, it's free. This book provides an introduction to the Obsidian app and walks you through a handful of key concepts to help you master the software. And the book presents several techniques shared by the wonderful Obsidian user community that will help new and experienced users alike to master this powerful software.