“Man’s Search for Meaning” tops my non-religious literature list absolutely. Frankl is a master interpreter of the human condition. “Outliers” is amazing. It felt like I was reading a biography of a recent past life. I very much identified with it, “David and Goliath” as well.
I would add, “See You at the Top” by Zig Ziglar. Not because of the content so much as the premise. So many modern books on self-development focus on modifying behaviors when in the presence of others instead of “being” the right kind of person to get the right kind of work to enjoy the rewards in life that you want to have. Behavioral changes miss the mark of changing the character and lead to compartmentalization. Changing your nature is harder but infinitely more valuable in all areas of success. Zig helps with defining what that looks like.
I would also add, “The Law of Success.” I don’t agree with all of what he has to say and Mr. Hill jumps off the deep end at a few points in the “mastermind” sections but generally solid. I had a 2nd edition set for many years. I believe it was 6 or 7 small books.