A few things come to mind, that I particularly liked but as everyone else already stated, self-help is a highly individualistic endeavor. So it depends on you and also the topics you are interested in. I do not like self-help books that tell you what to do but rather explain how things are and let you figure your design out by yourself. With that disclaimer:

• Why we sleep - Matthew Walker: (thanks so much, I found this gem recommended somewhere here on lemmy and it was eye-opening/eye-closing for me). This should be taught in school but sleep is basically highly underrated in our society.
• The mind illuminated - Culadasa John Yates: IMO the best book on meditation without all the esoteric stuff
• The subtle art of not giving a fuck - Mark Manson (not really that amazing, but it was the right thing for me to read at some point)

Not sure this qualifies as “self-help” but more as “helpful”…