I am a firm believer that you can't write about something you don't already have in you. A writer writes and creates out of the abundance in the well of his or her own soul. What is inside will be displayed on the page. If you have a deep soul, you will have a deep book. If you have a wide variety of experiences then your work will reflect that.
In other words, a book is a direct product of who the writer is. When I write I have to dip into my soul and spread it on the paper. Sometimes I do this knowingly, but most of the time this happens without me realizing it. I am just writing about life and characters in the way that seems most real to me. But I have to use what is inside me to create otherwise the story will sound flat, the characters will be one dimensional and the book will lack the one thing it needs: a connection to reality and truth.
So what do you do if you want to write a deep book, a book with soul?
Cultivate you. Cultivate your character. This is an art that our culture has lost, yet even so we all have character. Sometimes if we look at ourselves objectively we realize that our character is not what we would like. We may think we're the good guy, the main character, the hero, but we act and think like the villain.
Often as writers we can be so concerned about our writing skill and developing technique that we forget writing is the product of a person. Skill does have something to do with it and technique is important, but the person writing the story is more important. Take the time to develop yourself as well.
Discover who are you. When was the last time you stretched yourself and tried something you knew you would hate or find boring? All of life is open to the writer's scope. How can you write effectively about boredom or pain unless you have lived them? Do hard things. Learn new skills. Be really bad at something and then watch how you grow. Touch a leaf with your eyes closed. Try to describe the taste of milk. Climb to the top of a building using the stairs and see where you burn. Face your fears. Live.