There's a lot of hype around impact investing and also a lot interesting stories from social impact bonds (SIBs) to venture philanthropy. Below are a few interesting links that will help you find your way around this relatively new area:

Probably the most comprehensive work so far [this link was made in 2015] on social impact investing and definitely the best place to start:

http://www.oecd.org/sti/ind/social-impact-investment.htm

[UPDATE Nov 2017: I remember this document could be downloaded once but not anymore, at least at the OECD. You have to now purchase or read it online]

Social impact bonds

The best place to start anything on social impact bonds (SIB) is Social Finance UK, who launched the first SIB at Peterborough with the UK government way back in 2010. (They've just revamped their website on their 10th year anniversary so you should be able to find what you want quite easily).

For those with interests from a financial point of view, I regularly refer to the following documents from Social Finance UK :

A technical guide to developing SIBs

Evaluation of SIBs

This is a really interesting discussion paper on business modelling loneliness (a SIB was launched later on in the UK to tackle loneliness). The paper was prepared by Social Finance UK and supported by Calouste Gulbenkian Foundation (whose arm in Portugal have been part of the SIBs launched over there) and the Centre for Social Action Innovation Fund, a partnership between Nesta and the Cabinet Office UK.

The OECD has a working paper on SIBs (from 2016 - not sure if there is a final version somewhere).

A technical guide by MaRS Centre for Impact Investing in Toronto. This document has a great list of learning resources in the appendix.