You have already learnt dominant 7th chords earlier in the book. It is common to substitute dominant 7ths for other chord types, particularly in Blues and turnaround progressions. Shown below is a 1 6 2 5 turnaround where all except chord 1 are dominant 7ths. Learn this example and then transpose it to all other keys.
The following Blues progression contains a turnaround similar to the one shown above. Run through the basic progression first using arpeggios starting on the root note and then try improvising with the notes of the arpeggios as shown in the following example. Learn this solo in the key of Bb, analyze all the chord tones and then transpose it to all other keys.