Avinoam Fraenkel's Book The Introduction To Kabbalah has been a life changer for me.
IMHO it should be required curriculum in all Yeshivos and should count for infinitely more credits than some of the other utterly useless things they teach. All the while expecting underfed and fatigued teenage brains to sit quietly and take notes on the very things that they are taught the rest of the day have no intrinsic value but are rather a means to an end, while it's very clear to them that there the knowledge they are being imparted is of no other means than to obtain their diplomas. (Of course it's technically the right thing to do, but it's not very realistic.)