Book that has had positive impact on your life

Hello.
I'm looking for a book that you have read, and that book has had a positive impact on your life. A book that will change the way you think, but at the same time, it's interesting and not monotonous. I like to read only fiction and get easily bored by mainstream motivational book suggestions, so I'm looking for something apart from those.
Ps. Suggest for someone who is trying to change.. have a positive outlook on life after years of being a pessimist