The 13th Spy by Valerie Moolman
An American agent in Moscow crashes into the huge GUM department store. His body is full of deadly poison, and his briefcase contains every secret of value to Russian Intelligence. The records were typed out on paper from the American embassy. Worse yet, some of the information has turned up in secret files in Peking. The Russians demand American aid and accuse them of the leak. Enter Nick Carter, disguised as Tom Slade, disguised as Ivan Kokoschka, a Russian novelist. Nick discovers a ring of Chinese agents operating as the Brothers Twelve. They use a miniaturized light-activated listening device literally drawn onto a painting, to gather the secrets. The 13th Spy also introduces a new secondary character that will appear down the road in The Weapon of Night. The new character is named Valentina Sichikova.
The first (May 1965, A139F) and third (August 1966, A139F) printings have the same cover illustration. The only difference is the stated third big printing.
The next two state the same 5th printings but have different book numbers (A942S) and (AN1100). Looks like Award put stickers on some of the titles and reissued them. Awards did this one a few that I have in my collection. Fraulein Spy is one example. The left book cover was my second Nick Carter book purchase during a book sale in my hometown mall.
Last but not least are the UK editions from Brown Watson Digit House an April 1966 reprint and a 1970 Tandem edition reprint.
The next Nick Carter adventure is The Eyes of The Tiger by Manning Lee Stokes