Lightning
Yeah, Yellowfang's Secret was really disappointing. Sagewhisker is one of two medicine cats I dislike. She manipulated Yellowfang and pretty much straight lied to her. The power thing came out of nowhere and disappeared once it served its purpose. Also, Raggedstar is an abusive jerkwad and is never once called out on it.
This in a nutshell is exactly why I don't like Yellowfang's Secret. She was basically forced to become a medicine cat, and that's a boring thing to read about. If Yellowfang had wanted to become one, that's cool, but she really didn't, and she wanted to be a warrior. 