Not so much a life changer but certainly reshaping the way I look at things, the way I judge things, and the way I let things ruin my day:

'The Subtle Art of Not Giving a F*ck' by Mark Manson

The book is full of examples and while reading, you go "hey, that's a bit like my friend X", or "hey, that sounds like my Uncle Y". And then, when Mark describes certain personality traits, you think, "hole smoke, that's ME!"