One of my super short reviews on the books I read in my My Way to PHP series.
Elemental Design Patterns by Jason McColm Smith
What are design patterns made out of? This book gives the answer. Jason McColm Smith shows the fundamentals of OOP languages called Elemental Design Patterns 
(EDPs).
The book begins to shows the fundamental entities in OOP languages and then looks at their relationship. The result is EDPs.
He also introduces PIN (Pattern Instance Notation), a simplified version to describe patterns visually.
The book features a catalogue of the EDPs in the format of GoF. It also shows intermediate patterns which help to explain the design patterns you learned in GoF.
All in all, a really good book which helps you to see the underlying themes in design patterns and understand the fundamentals on patterns in OOP. Recommended!