An investment in knowledge always pays the best interest, as Ben Franklin said.
However, this is not a question I can answer for you, as it depends on the opportunities that are specifically available to you as an individual. Sometimes opportunities will knock on your door and you can take advantage, other times you have to create that door to allow opportunities to knock.
Maybe you have a friend that is opening a side business, maybe there is a class you can get into at a trivial cost. What I suggest is to start investing just to get into the habit of it, not so much for the returns. Before you do, however, any financial advisor will advise you to begin with a emergency fund, worth about 3-6 months of your expenses for that time. I wanted to hit the ground running and start investing in stocks, but first things first I guess.
"Millionaire Next Door" will help you get into a saving mindset, "I will teach you to be rich" is ok, plenty of other books. My advice is keep doing what you're doing, learn to start saving, and once you have obtained an emergency fund of the amount of your choosing, start looking to invest in Index Funds or ETFs through any platform that has LOW FEES!!
I use Betterment, but Vanguard is good too, as they allow you to get your feet wet and it's passive. Hope this helps.