Asembly and Hardware

Easy 6502 (opcodes)
Famicom Party
RP2040 Datasheet
RP2040 Assembly Language Programming
The Elements of Computing Systems

Forth

Starting Forth
Thinking Forth

C

The C programming Language (2nd edition)
C Programming - A Modern Approach

High-Level Languages

HTML Beginner Tutorial
Eloquent Javascript
Javascript Tutorial
Learn Python 3 the Hard Way

Misc. Programming

CFD Python
Introduction to Algorithms
Making Embedded Systems
Reverse Engineering for Beginners
Structure and Interpretation of Computer Programs

Math & Physics

Calculus
Linear Algebra
A First Course in General Relativity
Quantum Mechanics: the Theoretical Minimum
Quantum Field Theory in a Nutshell