You can find Driscoll's discussion of the development of his reflective cycle in Driscoll's Practising clinical supervision on page 43.
Your Harvard style in-text citation would be Driscoll (2007, p.43) or (Driscoll, 2007, p.43).
A full Harvard reference for your reference list would look like this:
Driscoll, J. (2007) Practising clinical supervision: a reflective approach for healthcare professionals. 2nd edn. Edinburgh: Bailliere Tindall Elsevier.
If you use a different referencing style, see the Referencing page for more guidance.
