Most of us photographers become gear heads and acquire way more gear than we ever need. While this may not necessarily be a bad thing, none of this gear will likely make us a better photographer. Of course there are a few exceptions to this but making an investment in knowledge will always make you a better photographer. Period.
In comparison to a new lens, a new book or an online course costs next to nothing! Heck, even having a binge on YouTube will be a worthy investment in your time, and will cost you nothing!
Depending on your end goal, the knowledge you pursue may not even be photography related. If you’re planning on making a living from photography, an online course in business would be invaluable. If you work with people alot, then maybe a little psychology would benefit you. The point is these investments will make you grow as a person and a photographer so much more than a new lens or shiny new camera ever could.
Your next photography investment should be in yourself!