There is a lot with Materials I’m still learning. I am no real expert in that area per se. There are plenty of users on the forums who may have a much wider wealth of knowledge than I do though.
For instance, Eric Ketchum (one of our Support Engineers) is great with materials and general VFX.
mAlkAv!An has, to me, been the Materials guru on the forums. He’s always posted a wealth of knowledge when someone gets stuck.
Eric has mentioned William “Bill” Kladis on numerous occasions. There is a lot of information in his UDK materials books that will carry over, but of course some information is outdated based on the rendering system that UE4 uses now.
If you post in the General Discussion section of the Forums there are probably a long list of information people can link you to to learn materials much better! 
Tim