The Well-Grounded Rubyist, Second Edition (2014)
Part 1. Ruby foundations
Chapter 1. Bootstrapping your Ruby literacy
Chapter 2. Objects, methods, and local variables
Chapter 3. Organizing objects with classes
Chapter 4. Modules and program organization
Chapter 5. The default object (self), scope, and visibility
Chapter 6. Control-flow techniques
Part 2. Built-in classes and modules
Chapter 7. Built-in essentials
Chapter 8. Strings, symbols, and other scalar objects
Chapter 9. Collection and container objects
Chapter 10. Collections central: Enumerable and Enumerator
Chapter 11. Regular expressions and regexp-based string operations
Chapter 12. File and I/O operations
Part 3. Ruby dynamics
Chapter 13. Object individuation
Chapter 14. Callable and runnable objects
Chapter 15. Callbacks, hooks, and runtime introspection