Primary Resources
Warning: Be aware that HyperPhysics doesn't always indicate the vector nature of certain quantities and expects the reader to know from context if something is a vector. Remember, forces always have a magnitude AND direction and must always be added using vectors.
Openstax textbook section on Friction.
Secondary Resources
Gives a pretty good representation on how to think about friction.
The next two videos, Doc Schuster explains Friction
Other Resources
A concise definition of friction with proper notation.