Secret Agent K-7

The introduction reads:
       "The stories in this book are told by a former member of the United States Secret Service, known as K-7.
       His adventures in the air as well as on land and sea in many countries are thrilling documents of the intrigue and espionage that exists today among the nations of the world.
       Modesty compels the hero to disguise himself under another name, K-7; but only the principal actor in such gripping dramas could report them in such detail."

       In the book, each adventure concerns the action of the same man, Secret Agent William Malone, although he does mention working with K-7 on a couple of instances. Malone is assisted by an old teammate, Joan Randall, in the first mission and by Yvonne Durrell in the others. [In the radio programs from which these are said to have come, none of the "special agents" are identified other than by codename but Agent M was the one who always worked with Yvonne.]

Six stories are told:
Saving the Suez - Secret Agent Bill Malone teams up with an old assistant, Joan, as he works to find who is planning on blocking the Suez Canal and how. He quickly learns it is an old adversary, a spy names Ostov.
Invisible Ink - [plot unknown - my copy had missing pages]
Sealed Orders - Malone is working with Yvonne to find out who was behind the car accident that put a leading naval officer carrying vital secrets in the hospital.
Enemies Within - Malone is asked by a high ranking cabinet member of a friendly nation to find out who is smuggling guns to arm rebels in his land. Yvonne is on hand to lend aid.
Secret Airfields - Malone is working again with Yvonne to locate those who have been building secret airfields near the Panama Canal to make destroying it easier.
The Code Book - The heartbroken love of a young officer convicted of stealing secrets and executed has offered her services to his spy boss. Malone has Yvonne go undercover as another woman with access to classified intel.