I don't have one book to put in this category, but I do have several that inspired and guided me through different phases of my life.
The Little House on the Prairie series and the Wizard of Oz series turned me on to a love of reading and getting lost in reading.
Beverly Cleary prepared me for childhood.
Judy Blume prepared me for falling in love lol.
Catcher in the Rye, Roll of Thunder, Hear my Cry, and Diary of Anne Frank opened me up to the harsh realities of the world and emotionally cushioned me through the emotional toil of my teenage years.
Siddhartha, Tao of Pooh/Te of Piglet, Way of the Peaceful Warrior, The Natural Mind, Hope for the Flowers, and Letters to a Young Poet inspired me throughout college & continue to do so today.