Worried about what's next
Books commenters say helped with worry, anxiety, or not knowing where to go.
Books several people mention
Ranked by how many different people say it helped. Small numbers, so the order is a hint.

A short, lyrical novel about a young man's search for spiritual enlightenment in ancient India.
This was a book I read in my early twenties that really helped me come to terms with a religious upbringing that no longer resonated with me, as well as a spiritual void in my life that needed to be filled.

Techniques for changing negative thinking habits to become more optimistic, resilient, and happier.
I read the book and did the work (similar to Cognitive Behavioral Therapy) and can honestly say it changed my life. I went from catastrophizing just about everything to not worrying pretty much at all.

An explanation of how anxiety works in the brain and how to calm it.
Knowing the mechanisms and biology behind those things helped me see the bigger picture and answer a lot of the "whys and hows".

A spiritual guide to ending mental suffering by living fully in the present moment.
the book showed me that I was worrying and thinking way too much and that this is at best useless and sometimes even dangerous.

An economist's contrarian case that parenting is easier and more rewarding than people assume.
I've definitely changed my attitude as a result and it's made parenting much more pleasant. I'm not totally checked out, I'm still doing the best I can, but I'm not as anxious about how what I'm doing affects my daughter.

An argument that building rare skills, not following your passion, leads to work you love.
The book completely changed the way I look at work/career/etc, and I wish I could have read it 20 years ago. It finally freed me from the catastrophe of "follow your passion".

A reflection on our short lives and how to spend time meaningfully, not just efficiently.
For me, the tools in the above books have helped me get a better idea of what I want to spend my time doing and how I'm going to try and get there.

An explanation of what charisma is and how anyone can learn to build it.
The main tool that helped me overcome it was the book The Charisma Myth. Can't recommend it enough, it literally changed my life.