The Game by Neil Strauss. I've not turned into a PUA after reading it (it was not the goal) but just after reading the first chapter, I just stood up from my desk and went straight to speak to a girl I was attracted to (but to which I was afraid to speak). I guess that can count as "altering my character".
I also second A Guide to the Good Life by William B. Irvine. Applying some of the stoic principles (negative visualization, dichotomy/trichotomy of control, fatalism, ...) helped me a lot to be peaceful with myself.
I also second A Guide to the Good Life by William B. Irvine. Applying some of the stoic principles (negative visualization, dichotomy/trichotomy of control, fatalism, ...) helped me a lot to be peaceful with myself.