Self-improvement is a realllly general term.

If you want to improve yourself, just improve yourself. Set a few goals for yourself like "I want to manage my money better" or "I want to be a better partner for my partner." Not too many goals; you can always think of more later. Then research how to achieve each topic.

Some of those topics may be found in, or even on the cover of self-help books. You can also just type it in google and see what happens there. You'll have to stumble over a few useless search results, but that only compensates for all the fluff you won't have to wade through.

My main point is, don't just buy self-help books if their goal does not align with one of your own more immediate goals. Get specific information that can actually help you. The extra worth of a good self-help book over sleuthing on the Internet is probably that it can help with building motivation.