If you rave about a book you just read, you're telling people you think it's great, or you're giving it a rave review.
Rave has two levels, one that implies you're very excited about something and the other that you are too excited, so you seem crazy. While raving about a movie means to talk it up, being a raving lunatic means you are engaging in a lot of crazy talk. When you rave about a performance, you talk enthusiastically about it. Take that enthusiastic talking to the next level, and you might be called raving in the crazy sense.