The advent of microfinance and microenterprise means mentoring must adapt to changing times. The Aspen Institute's FIELD program documents the latest data on how microbusiness sectors adapt. Aspen has tons of other programs too numerous to name here that can help. Mentors who need ROI trackers for their board service and volunteerism need look no further than True Impact.
A web search of topics covering mentoring reveals a ton of open-source research on personal mentoring for disadvantaged youth. The research on mentoring within a business context is often behind the paywalls of academic journals or held in the private databases of consultancies. The best open-source business cases for mentoring are in the Society for Human Resource Management's research and tools. Searching SHRM's site for variations on the word "mentor" reveals everything a good manager needs to know.
The existing literature on mentoring has some gaps. Mentoring disadvantaged youth makes sense from a humanitarian standpoint. Finding a mentor at work has potential payoffs in the time-honored tradition of riding a superstar's coattails. The HR coursework I recall from my business studies showed that formal mentoring programs often lead to mismatches. There is room for disruption in mentoring. Social media enablers like MicroMentor and Startgrid are natural evolutions in business relationships.