The basis of diatonic harmony ( harmony derived from the major scale) can be very easily understood by plotting the C Major (easiest to understand because there are no sharps and flats) scale all over the 12 fret x 6 string grid.As you proceed to play the tones along each string you will just notice that they are just linear displacements of the other strings based on the tuning of a guitar.So at any point along a fret you will get some cool combinations of notes in the scale.
The chords derived from this scale are: 1Maj,2Min,3Min,4Maj,5Dominant,6 Min,7 Half dim.
CMaj,Dmin,Emin,FMaj,GDominant,A Min,B Half dim.
The spelling of a fundamental Major7th chord is (1,3,5,7),Min7 (1,b3,5,b7),Dom (1,3,5,b7),Half Dim( 1,b3,b5,b7).
The exercise is to circle these groups on the grid and you will have all inversions of all these principal chords without any fancy alterations. This is the basis of the harmony implied in the major scale.You will find crazy large intervals that you can use, and close intervals as well.Circle away and study this circled chart focusing on each chord in the family.e.g. Study the Dm7 in all its forms on one day etc.
Gradually the logic that lies within the scale will be very understandable. By taking again diads,triads and chords for each component you can easily learn this very important concept. No need to break your head over a book that kills you with rhetoric and repetition. Yes advanced concepts can be broken down much easily once the fundamental building blocks are down.
Please plot and verify and totally enjoy :)