For more studies, here is a list of abstracts compiled by the Institute of Coaching, to answer the question “Does coaching make a difference?” Founded in 2009, and based at McLean Hospital at Harvard Medical School, the Institute seeks to enhance global well-being through science-based coaching, and offers professional development, research, and a vast coaching library.
You can also refer to the book below, which compiles 15 peer-reviewed chapters from leading experts, presenting coaching within contemporary theory and research, especially key presentations from the 2003 Evidence-Based Coaching Conference. The aim of the book is to develop a knowledge base and informed dialogue among coaches, emphasising scientifically grounded executive, personal, and life coaching.
Evidence-based coaching, Vol 1: Theory, research and practice from the behavioural sciences.
Cavanagh, M., Grant, A. M., & Kemp, T. (Eds.). (2005). Australian Academic Press.
Finally, the International Coaching Federation (ICF) has a research portal where you can search for more studies based on keywords, read abstracts and download full papers.