The Power of Now by Eckhart Tolle. I had pretty much given up on the self help genre but a comment left by a poster on Youtube video led me to the book. Something about the title grabbed me and I read not only The Power of Now but 3 of his other books as well over a 3 week period in December - all of which strongly resonated with me. I live my life - really each present moment - differently because of Tolle's teaching. So my life changing books for 2015 - all by Eckhart Tolle were:

The Power of Now; Practicing the Power of Now; Stillness Speaks; A New Earth