External resources
Here is a collection of learning material from third parties that might help you master the language.
Crystal Programming
Crystal Programming is a project-based book by George Dietrich and Guilherme Bernal. It covers all the fundamental concepts needed to start programming in Crystal such as OOP and concurrent programming.
Crystal for Rubyists
If you know Ruby, you are just a few steps from learning Crystal. From Core Team Member and Kemal creator Serdar Dogruyol comes this guide that will help you to transition from Ruby to Crystal.
Exercism
Get your hands on Crystal with a series of exercises, and compare your solution to that of the many members who tried already.