Packt has many other great resources for learning about SOLID principles: https://www.packtpub.com/mapt/book/application_development/9781787121300/1 https://www.packtpub.com/mapt/book/application_development/9781785884375/10/ch10lvl1sec50/the-solid-principles https://www.packtpub.com/mapt/book/application_development/9781785280832/8