Robert Arthur Jr. - The Mystery of the Whispering Mummy

Nothing at all suspicious about the chap in the hat or prize cat

Moving on from the Famous Five, I discovered another series about young sleuths, older boys this time and more focussed, solving mysteries with a combination of daring and brainwork, and getting the better of adult baddies. Fictional friends that I could imagine collaborating with and feel that urgent earnestness of boyhood.
 
Although The Three Investigators series was presented by Alfred Hitchcock, he didn’t write any of them. The Mystery of the Whispering Mummy was by lead author, Robert Arthur Jr., while The Secret of the Crooked Cat was by William Arden.