Richard Marson's indeed "ultimate" books about Upstairs, Downstairs, LWT's superior period drama produced between 1971 and 1975, are a perfect blend of competent film analysis and marvellously detailed production history based on interviews. A role model if ever I've seen one. Almost impossible to match, they set the standard for other TV books VERY high.
Being a TV insider himself, Richard probably gets more answers than your ordinary researcher: he managed to extract comments from virtually every key programme contributor both in front of and behind the camera(s).
For the revised hardback edition he tracked down lots of crew pics, thus ameliorating the, perhaps, only weakness of the original Kaleidoscope paperback: lack of photographic material.