I'm a beginner guitarist. Currently I'm working with "A Modern Method for Guitar: 123 Complete" by William Leavitt. Looking for some good 101 textbooks for music theory.

After completing the first chapter of Leavitt's book, I have a feeling that I'm only learning to play the guitar. But I know very little about the theory behind the music.
For example, the very first thing I learnt is the C major scale. I can play the scale, but I don't know what scale signifies and how it's related to the C chord. Also there're variations of C chord [E G C E'] & [E G C F'], how do I know what to play with only the chord name is given?

Sorry if the above questions are difficult to understand as I'm not familiar with describing musical elements.

Thanks,
681726

EDIT: correct grammer