Guide tones are simply the third and seventh of each chord.
The guide tones for example on a IMaj7-vim7-iim7-V7 in the key of C major
would be EB,GC, FC, and FB. Many theory/practice methods (Jamey Aebersold's
books and Emily Remler's videos come to my mind right now) stress the need
to compose "guide tone lines" which have close voice leading. As an example
of a line on the above progression: B,C,C,B. The idea is to compose
melodies containing chord tones which could stand on their own without
accompaniment without just arpeggiating each chord.
If the guide tones are placed at the occurence of each chord, on 1 and 3 of a
measure with 1/2 note harmonic rhythm, the player or composer can stray quite
far outside the harmony on all other parts of the measure while still having
the guide tones to clue the listener in to the harmonic movement of the tune.
A great example of a iim7-V7 melody with mostly guide tones is "Tea for Two,"
the first three notes of the melody in C are C,A, and B. The C is the
seventh of the Dm7 the B is the third of the G7 chord and occurs as the chord
is played.
The blues, "Tenor Madness" by Sonny Rollins is full of guide tone-linked
lines.
A book I am really enjoying which adheres to this approach is, "Connecting
Chords With Linear Harmony," by Bert Ligon ( a frequent contributor to this
list).
As a guitar player, I often use guide tones as a counterpoint to the melody
or solo to accompany myself and strengthen the ensemble's harmonic base when
I am working with a bassist and no pianist. This practice can be heard quite
frequently on recordings by Jim Hall, Mike Stern, John Scofield, John
Abercrombie, Jimmy Bruno, John Stowell, and other guitarists who work without
piano players in trio and duo situations.
I hope this was helpful!
Brian Oates