Mine will be none. Only practice, coding, building, testing, exercising again and again has helped me.
I worked through The Little Lisper when I was at university and I got a lot out of it, for example.
Can you recommend any other content like that?
Still, a couple of random things I can think of:
The Spatialite Cookbook is no longer maintained, but I think still useful. That's structured as a sequence of fun exercises: http://www.gaia-gis.it/gaia-sins/spatialite-cookbook/index.h...
I also enjoyed Peter Norvig's Design of Computer Programs online course: https://www.udacity.com/course/design-of-computer-programs--...
But in general - if you are just trying to become a better software developer (e.g write clean, well tested code) you are absolutely right no book will get you there, hard work will.