Not sure if it counts as a self-help book, but Unmasking Autism. Also Laziness does not exist by the same author.

I liked Four Thousand Weeks. Reject capitalist modernity’s obsession with getting everything done, stop cramming your life with life hacks, stop trying to maximize productivity, kill the manager in your brain, accept that your life is only a few thousand weeks long, and make the most of the limited time you have.

“Feeling Good” in part helped me with depression. Really it was cognitive behavioral therapy as a practice, but the explanation in that book got me started with the help of my therapist.

A therapist gave me excerpts from Jon Kabat Zinn’s Full Catastrophe Living and it changed my life.

A Guide to the Good Life by William Irvine also had a huge effect on me. I use a number of the stoic exercises daily, 12+ years after reading the book.