I'm a fan of self help books. A lot of them are rubbish but the ones written by doctors and therapists are often very good. I am interested in psychology so I will read them for enjoyment and education as much as for self improvement.
I found these very useful when I suffered from anxiety after having some major health issues:
Self Help for Your Nerves by Dr Claire Weekes
Stop Thinking Start Living - excellent CBT book on negative thinking by Richard Carlson (all his books are good)
The Worry Cure by David Leahy and
Feel Good New Mood Therapy by David Burns, both good CBT books.
7 Habits of Highly Effective People is pretty good. Anything by Gretchen Rubin is good; her books on the 4 Tendencies (personality types) and habits are both a good read.
I'm currently reading Atomic Habits by James Clear which has some good strategies for habit building.
Daring Greatly by Brene Brown is very good. It's about being prepared to show vulnerability.
Two books that many people rate, but which I thought were pants, were The Power of Now by Eckhard Tolle - just a knock off of Buddhist ideas, nothing original, very lightweight - and Reasons Not to Die by Matt Haigh - really piss poor and lightweight, just a very simple account of his struggles but with not much practical help. Mad Girl by Bryony Gordon is a much better (and funny) account of dealing with mental health issues.
If you look on Abebooks or Amazon you can get them used fairly cheaply. Also, The Works usually have some stuff for a couple of quid.
I also quite like the Zen Habits web site, and the Tiny Buddha web site (it's just articles; not about Buddhism). I like learning stuff so I'm always interested in new ideas and insights.