A massive amount of notes and links about unreal, c++, and many other languages and technologies. The unreal notes doc was a good first read when starting to dip past the surface of the engine.

Condensed and thorough documentation on multithreading and general performance.

Orfeas’s site is another gold bookmark with many unique c++ tutorials for Unreal, covering some uncommonly found topics.

One of the most important Unreal C++ learning sites around. Tom’s work can teach you everything you need to know about Unreal.

Formerly benui.ca — a learning site with a huge number of great Unreal tutorials and resources, mostly focused on UI, but covering a variety of topics in the engine.