Geometry: Seeing, Doing, Understanding
This book helped me a lot on how to understand geometry. The diagrams and pictures are immensely useful. Fun and related to the topic, the comics are quite entertaining.
The problem is though, the answers in the book
are often wrong. Also, the instructions on how to do the constructions are a bit hard to understand. Despite that, this book along with my teacher made geometry one of my most favorite subjects.
And, importantly for self-learners, a comprehensive solution manual is available with answers to all ~5000 exercises.