I followed up my recently renewed enthusiasm for The Prisoner by getting hold of several of the books about it, and this seemed the best starting point: 135 pages of The Prisoner: The Official Companion to the Classic TV series, by Robert Fairclough, with a (rather incoherent) foreword by Kenneth Griffith in the absence of McGoohan. It's a good basic guide to how The Prisoner came to be made, six pages of info on each episode, and then some follow-up chapters on its impact at the time and later influence, both on popular culture in general and on Portmeirion in particular. It certainly made me want to go back and re-watch a couple of the episodes which I didn't properly appreciate last time I saw them.
The story that isn't told, but which I imagine I'll find elsewhere, is how and when relations between McGoohan and his sponsors deteriorated to the point where the plug was pulled, with only 17 of the planned 26+ episodes being made. Likewise, some pulling together of how McGoohan drove the creative team (including himself) insane would have been usefully illustrative. (And why did the very first Number Two, Guy Doleman, unexpectedly leave the filming several days early?)
I was also interested to note that The Prisoner was shown over all of England and Scotland, but not Wales or Northern Ireland, in its first 1967-8 run. I wonder if there was any particular reason for that?