Discovering Geometry Online Book 

Chapter 1: Introducing Geometry
Chapter 2: Reasoning in Geometry
Chapter 3: Using Tools of Geometry
Chapter 4: Discovering and Proving Triangle Properties
Chapter 5: Discovering and Proving Polygon Properties
Chapter 6: Discovering and Proving Circle Properties
Chapter 7: Transformations and Tessellations
Chapter 8: Area
Chapter 9: The Pythagorean Theorem
Chapter 10: Volume
Chapter 11: Similarity
Chapter 12: Trigonometry
Chapter 13: Geometry as a Mathematical System