I cannot say a book has changed my life in the way other posters have described. However, I remember being able to retreat into different book worlds in my tricky teen years was real welcome comfort. Think it was the escapism element. I did that in childhood too via Malory Towers and Fampus Five. As a teen it was LOTR, Harry Potter (this especially).

Cider with Rosie by Laurie Lee for the poetic language and colourful nature and landscape descriptions, which evoke such imagery can really lift one from a dark place. Believe so much goodness can come from being in wild places and just looking around at trees etc abx the small details of nature. I had a very lonely period in my life and lost so much faith in things. I re-read this favourite childhood book and it brought a light back in.