I'd say one of the most profound books I read was John Bradshaw's, "Healing the Shame that Binds You." I had been locked in shame for years and really had no idea that's what was happening. Once I read the book it was like a tadpole realizing it lives in toxic water and the transition to land and clean air has always been it's choice to make. I had already been into self-improvement for quite some time, but that was the first book that guided me to emotional wellness.
I am kind of cheating with this one because it's not a book, but, Leo's Ultimate Life Purpose Course is probably one of the best self-improvement programs out there. It will give you such an immense degree of clarity and purpose that you'll be like a magnetic arrow flying through life attracting only the things that serve your life purpose. I did it at the beginning of quarantine and it's fucking spooky how the right opportunities, the right book, the right conversation, the right trip, have just appeared in my life. The percentage of Leo's viewers who haven't done the course is probably terrifyingly low for how powerful it is.