Procedural Generation in Game Design - provides a good overview, but is a bit light on implementation details. https://www.amazon.com/Procedural-Generation-Design-Tanya-Sh...

Mazes for Programmers - only covers procedural mazes, but contains a lot of code and different types of mazes. https://www.amazon.com/Mazes-Programmers-Twisty-Little-Passa...

Great channel with Unity tutorials on terrain generation, cave generation using cellular automata etc: https://www.youtube.com/channel/UCmtyQOKKmrMVaKuRXz02jbQ

Procedural Content Generation for C++ Game Development - decent, but basic book with 2D examples in SFML. https://www.packtpub.com/game-development/procedural-content...

Procedural generation Reddit: https://www.reddit.com/r/proceduralgeneration/