Yes it's in Am with E7 as the dominant as explained above, very common pattern deriving from classical harmony (Greensleeves, Walk don't run etc). Whereas in modal music the harmonies tend to move parallel through the scale, the dominant sound is favoured by Blues players. A more unusual use of the E7 chord when in the key of C would be Freight Train and its siblings, where the chords move:

C G7 G7 C E7 F C G7 C