For perfectly understandable reasons, there is not a lot openly available on managing human sources for the collection of intelligence. However, the June 2009 issue of the FBI Law Enforcement Bulletin does have a decent piece written from the LE perspective that provides some simple, clear lessons with wide application:

(Note: The LEB does not provide links to individual articles; this article is on pages 3-11 of the linked pdf)

It’s All About Them: Tools and Techniques for Interviewing and Human Source Development
.....Because recruiting confidential sources for human intelligence collection constitutes a primary function of the profession and represents the key to any investigation—whether terrorism, counterintelligence, drug trafficking, gangs, or the myriad of other criminal violations—no professional law enforcement organization can succeed in its pursuit of securing the United States from all threats without this valuable commodity. Although the current professionals entering law enforcement are highly educated, technically savvy, and extremely intelligent, some have not had the opportunity to develop the human interpersonal skills that time and experience can provide. Compounding the challenge is the increasing workload that inhibits veteran professionals from devoting the necessary time to mentor incoming personnel.

One solution that can help alleviate the difficulty of having less time to mentor involves breaking down the practice of relationship building into clear, understandable steps and phases. An effective law enforcement professional and leader can take the “art” of relationship building and make it “paint by number.” To illustrate this concept, the author presents a realistic interview and a followup explanation involving a veteran law enforcement professional and his less experienced colleague.....