Which books, courses, projects have improved your programming skills immensely?

The F# for Fun and Profit[1] site by Scott Wlaschin is phenomenal as a general programming education site, not just for F# specific things (though there's plenty of that as well). The "Designing with types" series[2] is a good starting point.

[1] https://fsharpforfunandprofit.com/

[2] https://fsharpforfunandprofit.com/series/designing-with-type...

I enjoyed reading the book "Building Microservices" available at https://www.oreilly.com/library/view/building-microservices-.... The book covers various aspects modern software development, such as observability and resiliency. Despite its title, the book is not exclusively focused on microservices, and the author does not unequivocally assert that microservices are the ultimate solution.