In the mid-60s, the successful British publishing house of Wm Collins Sons added to their imprints the small-sized paperback "Spitfire" books. Measuring a diminutive 5" x 3.5" they were roughly halfway between Whitman's Big Little Books and normal sized paperbacks. Since the font size was standard, each page was half of what a regular book would have and with the books running just 120+ pages, each story was more a novella than a book. This made it a good fit for young readers.
A third Basil Deakin book in the Spitfire format, Secret of Zarb, was released at the same time and looks to be similar to the two Rod Renton books but was about a different pair of agents, Greg Grayson and Steve Stanton who work for an organization named W.A.S.P.