Two soldiers told me, so I guess word of mouth, but also, Tom Clancy talks about it a lot in some of his novels. Not that Clancy novels are found in the nonfiction section, but his stuff uses facts to form fiction-based story lines.
The two soldiers I talked to were an Army Ranger (who had himself trained with them) and former Special Operations Green Beret out of Bragg.
The Ranger talked about a cross-nation training session they had in Israel with the SAS, GSG9, Rangers, US SOCOM troops, and some French Special Forces.