For more info on Grand Funk please consult your local library!
A hidden gem of classic rock is Ry Cooder. He provided the lead guitar for the Rolling Stones` "Honky Tonk Woman" and is probably the best slide guitar player of the past 50 years. Into the Purple Valley and Paradise Lunch are probably his two best albums.
Also, definitely check out Creedence Clearwater Revival. This may surprise you, but they had a rather large impact on the foundation of indie rock, at least in ideas more than music.