The Case of the Frightened Friend — Robert Newman

Blink once for yes, twice for no, and three times for murder

Sometimes it seems like all the boys at school know about Andrew Tillet's friendship with Inspector Peter Wyatt of Scotland Yard. Mostly it's a little embarrassing, but when Andrew's classmate Cortland makes a very strange request, it becomes handy to have friends who can launch an official investigation . . . as well as ones who work in more unofficial ways.
 
Cortland thinks there's something not quite right about his grandfather's poor health, but they'll have to get the old man out from under the watchful eye of Dr. Thurlow before they can be sure. This calls for another ingenious scheme . . . and for keen-minded Inspector Wyatt to connect a retired admiral to a high-level intelligence leak. Someone is not above committing treason -- or murder -- and has no intention of going quietly.