I am looking for an introductory book on group theory for my son who is a high school freshman. He has a good grasp of the basics of mathematics and is ready to take calculus classes. He has a very strong intuitive grasp of symmetry and transformations, so I thought that he may be ready to be exposed to group theory. (I know I did when I was in high school, when I had a wonderful math teacher who was working on his PhD thesis while he was teaching and took the time to take the best kids in his class aside and teach them advanced math topics). Are there any recommendations for good introductory group theory books? We are going through some of the online resources, but I thought a formal textbook treatment may also be useful. Thanks in advance!