While it may not be the most important, if you've been meaning to up your terminal game and want a bit more dexterity when shuffling between panes, tmux and vim are a good area to invest time in [1]
Personality theories:
Textbooks in this area. Affordable on Amazon if you get them used, some are also on kindle but pricey.
An Introduction to Theories of Personality: 7th Edition by Robert B. Ewen is so good.
They can be used as sampler to springboard into areas of psychology you like. The material in this space - normally in paper books - is fantastic. It's totally normal to read through material from 50-70+ years ago.
[1] Not the configuration part, though. That's where the timesink is. Try to be basic as possible with vim configs. `vimtutor` is nice to play with.