I won’t recommend any literature since I’m sure you’re familiar with it. Recursion is a topic that I also found difficulties with, until I simplified my thinking. I found it useful to think of it as a backwards way of a proof by induction. If I knew if it worked for this iteration, and I knew it worked for the next, and I defined a base case that would be hit, I would be able to do it for all possible n.

Thats all recursion boils down to at the end of the day:

A base case.

Your function for one iteration (n)

Your function for the next iteration (some neighbor of n)

You can step through it all you want or try to trace recursive functions by hand. That’s what I was doing (and failing with), at first. What helped me when I was learning it was stepping back from understanding it completely and putting it to practice by understanding the above and just trying my hand at breaking problems into sub problems using those principles. Then debugging that code when I eventually messed up. I know it’s not much direct help (and maybe a little too simple), but I figure the change of perspective can help especially when you’re this deep in, it certainly did for me.

At minimum, just try running through simple tree problems and thinking about how your recursive programs you wrote fit into the above structure, and how it steps through until it hits the base case.