Some of the things I’ve read/watched lately to sharpen my skills:

• Inside the C standard library by Begriffs (and its predecessor on portability for weird machines)
• Making C less dangerous by Kees Cook (at Google back then)
• Streamlining systemd’s code and safety
• Algorithms by Jeff Erickson and the corresponding materials at the Illinois University course. I’ll get around later with a more thorough study of this one.
• A compiler writing journey
• The complete guide to (external) Domain Specific Languages
• HabitLab: Online Behavior Change Experiments