I seem to have been reading self improvement books for decades and that would be because it's true! I have certainly read a whole lot of books in my time and my first reflection in response to David Riklan's challenge is that every book I have ever read is about self improvement.

Every book I have ever read provoked reflection and re evaluation of my life, every book has opened some new dimension in more detail than I had before, every book has caused me to identify with a character either real or fictional that I had not met before, every book has exercised my empathy 'muscle'.

Anyway to respond to the question more directly there are five books that have impacted my thinking and actions more than any other, ever.

I have read all four in the past few years, a couple of them in the past few months.
In order of appearaance (on my Radar) then, Daniel Pink's 'Whole New Mind' opened up a vista for the future that resonated with everything I had learned and read about the immediate outlook for our (commercial) existence on the small blue planet. As a natural born right brainer perhaps I related more strongly to Daniel's projections but then and now it makes sense of where we are at on so many fronts. In an even broader sphere of self help for society as we know it, Jared Diamond's 'Collapse' provides a somewhat less upbeat prognosis but accurately and dispassionately describes greed and overuse of resources as the root cause of so many previous nation states falling apart; when will we ever learn? yes I know, that dates me and Bob Dylan!!
Jim Collin's 'Good to Great' and it's balance of empirical and qualitative analysis is for me is a bit like seeing the DNA of corporate success, for individuals and corporations alike the hedgehog concept linking passion,competence and prospect is surely what we all need to grab hold of. Especially in the current climate of change, what we are truly good at and passionate about is what will prevail in the long term.
If 'Good to Great' is the corporate genome then Michael Masterson's Ready, Fire, Aim is the Old and New Testament for all of us entrepreneurs; I do not believe there is a more practical, soundly based prescription for entrepreneurial success than this book any where, ever. 'The Secret' must have impacted many millions of lives by now and my secret is that I have not seen it; I have however read 'The Answer' by John Assaraf and Murray Smith and just ahead of extensive and fierce competition, it makes my fifth because it delivers clear prescription for us poor flawed humans to move forward. A check list and action points for us as individuals, broader but not better than Masterson's benchmark entrepreneurial planner.
Three books that give us a view of the future, two books that give us some clues on how we can better invent it!

Author's Bio: 

Crocodile Hunter, Farmer, Corporate Adviser, Inventor, aspiring author of 'Horizons', Real Life 102 and prerequisites, a future best seller.