The most complete and interesting Geometry theory book I’ve ever read is ‘Euclidean Geometry In Mathematical Olympiads’, by Evan Chen.
We could say that it’s the Bible of people who love Geometry problems in the Olympiads. It is divided into 10 theory chapters, which talk about different branches and techniques you can use to solve Geometry problems. There are a lot of theorems, with proofs, some instructive exercises with solutions and the end-of-chapter exercises, which are very challenging. If you can’t manage to solve them, some hints and solutions are provided at the end of the book. Also, there’s an eleventh chapter which only contains Evan Chen’s favourite exercises.
The book covers a lot of topics and techniques, both synthetic (homotheties, spiral similarities, …) and analytic (barycentric coordinates, complex numbers, …)
Very few requirements are needed to understand this book: essentially the basics of Geometry, like Thales Theorem, congruent and similar triangles, vectors. Don’t make the error of thinking it’s an easy book because anyone can read it, since it’s very difficult to completely understand all of it.
The chapter I enjoyed the most was chapter 9, about Projective Geometry and Harmonic Bundles. It was a new topic for me and I noticed it had so many applications I completely fell in love with it.
You can find my old article about Projective Geometry here: https://www.mattiagiuri.com/2020/04/24/an-insight-into-projective-geometry/
To sum up, I recommend this book to everyone who would like to gain a deep insight and have fun with Geometry in the Mathematical Olympiads, as it’s definitely the best manual you can find.