What will be the best and low-budget educational materials and platforms?
Learning materials and platforms as in books, simulation softwares, development kits (from which manufacturers?), affordable tools.

What I already know:
1. C and C++ programming.
2. Python
3. Built a few projects with Arduino
4. Raspberry Pi

Can anyone suggest educational, effective and practical ways to get myself familiarize in embedded system environment (in both hardware and software)?

Thanks in advance.