I have been looking at questions on Math Stack Exchange and I am frequently coming across topics that sound as if they could have been optional chapters in a high school geometry class, but I have never heard of them. "Radical circles", "Euler line", and so forth. It feels almost as if there were a Geometry II class that I somehow skipped, leaving a hole in my knowledge.
Note I am not looking for advanced courses like differential geometry or projective geometry, but rather things that look like challenge problems in ordinary high school geometry. In particular I would like to find a textbook for self-study. I own several high school "Geometry I" textbooks already. I would like a textbook for this hypothetical "Geometry II" course. Any suggestions are appreciated.