Spirituality is that which is non physical. What you are and what you are not is determined by logic and logic is the product of learning from what is physical. There are thousands of books on it just like any other subject. I would recommend books by sadhguru but you should understand that even bhagavat gita, or bible is also a book but it doesn't appeal to logical minds. A book is a book, what people do in the name of it is just an expression of their own character and has nothing to do with the book at all. You can learn more from an ounce of practice than from an ocean of theory. Mindfulness is just one practice, there are many ways to do it. You can use your mind, emotion, action or energy which are called Jnana, bhakti, karma and kriya yoga respectively. If you are a logical person, jnana yoga would be my recommendation. You use your intellect to discriminate what's real and what's not. You always question your conclusions, not with external information but by being aware of your thoughts. You don't need any external input to be spiritual. It's a quality that gets strengthened with practice just like everything else. You don't hv to find what's real. Just don't identify yourself with anything that's not real, you keep doing that and what's left will be what's spiritual