Free learning to shade course on Youtube
https://www.youtube.com/watch?v=mF3ZZo-Dbw4&list=PLI-9-SuuU7GE-swHeTgmnvmNb2cChOj_x
Dorian Iten is amazing, check him out!

This video and everything related in the sidebar is useful
https://www.youtube.com/watch?v=X0xL3cadSz0&t=209s

Best books

Light for the artist by Ted Seth Jacobs
Color and light James Gurney