I politely asked the owner of the book what was classified about something published in Flight. His reply? "The fact that we are using it!" 
As a general rule that is true as it gave credibility to a public source. 'We know they know we know, but do they know we know they know?'
We once had an open source Russian document that accurately set out the detail on our sensitive acoustic sensors one of which was never talked about outside secure rooms.