Of all the self-help books I read, Type Talk by Otto Kroeger and Janet Thuesen was by far the most confusing; not because it was hard to understand, but because it is hard to apply.
You see "Type Talk" takes an in-depth look at the Myers-Briggs Type Indicator: a personality test that is supposed to help you better understand yourself by enabling you to classify your personality into one of 16 types. It does so by teaching you a metric of 8 preferences revolving around how you see the world, interact with it, plan around it, and make decisions in it.
But people are very complex, and placing oneself in what is indeed a huge spectrum can be quite difficult. So as I write this, I am still confused about where I fit. But I certainly know more now, than I did before. So I would recommend this book for any young adult starting to discover themselves, and I suspect it would produce even better results with those further along in their lives.
Edit June 12 2017: Four years after the original writing of this article, I can stand behind my statements above. The MBTI is most definitely a useful and fascinating tool for figuring out one's self, as well as others. My main challenge with using it for self-assessment back then I think, was the dilemma of distinguishing who I was trying to be, from who I naturally am.
To apply the MBTI effectively (in a self assessment), one has to be totally honest with themselves, and as a slightly younger man there were still many behaviours I aspired to (and sometimes forced on myself), that weren't really me. So my answers did not reflect my natural state, and I eventually came to the conclusion that I was an ESTJ - which really was more rooted in aspirations, than in actuality.
Today, I believe that I'm an INTP (and in typical INTP fashion, I'm still open to new information - still wondering whether I may be an INTJ instead, or a male INFJ, or something else that I haven't even considered! Lol). But at least for now the shoe fits comfortably.
Still, this is all the more reason to read this book sooner rather than later. If it could take you this long to arrive at a tentative conclusion, the sooner you start asking yourself the question - the better. That said, knowing your type isn't about putting yourself in a box. Extroverts can be introspective, just as Introverts can be outgoing.
Knowing your type simply answers the question of your default settings - and these in turn help you to assess and modify your own behaviour in different contexts when you feel the need. So once again, I recommend this book - but not just for young adults, for anyone looking to find themselves.