Macklin had a number of books in the late 1960s thru the 1970s, and the only references I could find for him were a number from 2002 asking “Where is John Macklin?” Pseudonym? Still living? Abducted by UFO because he got too close?
Do you recall any titles? I have UFO/Paranormal stuff by Brad Steiger, the Colemans, Keyhoe, even Eric Norman aka Warren Smith (who I just looked up in case this was another pseudonym) & I’ve never heard of John Macklin.
Here’s a list from amazon.com. The only one that I personally have is “Strange Destinies.” (And I got it so long ago, I don’t remember any of the stories in it.)