There's only one "self-help" book I ever found useful. Not that I read a ton of these to begin with, but I got this one as a gift in my twenties, and I swear by some of the stuff included in it:

https://www.goodreads.com/book/show/15791144-adulting

It's really easy to read and digest, and what I love the most about it is that it gives you concrete things you can do to improve your life instead of rambling on about how you should view the world and yourself within it. There's no preaching here, it's a straight list of shit you can do right now to make your life better. Like buying a handheld steamer to get the wrinkles out of your clothes. That shit changed my life.