You don't need to make any of your functions stronger. Just be yourself. Your cognitive functions develop naturally as you age.
The only reason I'm interested in playing around with different cognitive functions and strengthening my weaker ones is so I can get along with different kinds of people. (So I can get promoted at work, get stuff done, and have more fun doing it on the side.)
I guess learning the different cog functions is like learning a language or a new culture, but fluency is much, much, harder and subtler. You can read personality type theory until it's coming out of your ears, but actually doing it - putting the functions into practice, especially your shadow functions - takes lots of observation and experience. I don't think that we're hard-wired to be our MBTI types, and you can train yourself to be something else in a matter of time. (But based on our developmental psychology, there are limits to how much you can change yourself, of course.) I guess improv comedians/actors are probably really good at this.
Basically, I'm doing this so I can get people-fluency in the same way that I have fluency in the English language and American culture. The book is a great idea.