This is where I keep track of selected reference reports that were interesting/inspiring, but I haven't written about separately (yet?) - this list is updated constantly as/when I discover new material. I hope this helps distill useful information for your exploration :)
Basics / Topdown updates
Unpacking the Impact in Impact Investing, Paul Brest & Kelly Born, SSIR (Apr 2013) - This is a great overview on fundamentals.
A closer look at impact investing, Vivek Pandit & Toshan Tamhane, McKinsey Quarterly (Feb 2018) - Excellent empirical review of impact investing deals in India.
"Requiem for a Dream - of an online impact investing platform", Astrid Scholz, ImpactAlpha (Feb 2018) - This echoes my views on the challenges and role of intermediaries in this sector.
Measuring the impact in impact investing, Ivy So & Alina Staskevicius, HBS (2015) - Bird's eye view on measurement practices.
The Power of Impact Investing, Judith Rodin & Margot Brandenburg, Wharton Digital Press 2014
The Impact Investor Handbook, Jed Emerson et al, Anthem Press 2017