It helped that I worked at Google before writing a book about the company, so I could talk about its leadership from personal experience and also knew whom to ask to fill in the holes when I needed more detail. However, I also had to walk a fine line to keep from violating the NDA in my employment contract.
Fortunately, Google was very forthcoming and allowed me to interview a lot of staffers, though they did insist on having a PR person present to record everything. In some ways I probably censored myself, perhaps unnecessarily. Steven Levy, in his book about Google, describes a technology called "Phil" that I had thought was top secret, yet he had no problems getting details about it from current Google staff. I'm not sure I could have talked about Phil, based on what I learned as an employee, but an outsider could write about anything he or she uncovered.
If you have no connection to the organization you want to write about, it's likely going to be hard to get much new information. You'll have to cultivate someone who does have that access or you'll be forced to rehash what others have said. How you establish that relationship is up to you.