The Spy and the Atom-Gun

"A first person account of a British agent [Geoffrey Martel, not Brian Grant] who slips into Gallonia to bring out a small atomic weapon of interest to the Western nations". The operative makes a few very favorable mentions of his fellow agent, Grant.
[Note: this is an unusual instance where one cover clearly says it is a Brian Grant operation and yet Grant himself does not show up! Martel does make three references to his fellow agent, Grant.]