I used the Mass-Observation archives extensively in my PhD research (see www.ww2poster.co.uk), as it has lots of really interesting material from observations (both direct and indirect) plus collated materials from the war years (and since). It was really ahead of its time! Much of the best material is only available by visiting the archives (based at the University of Sussex), but some of their published material is shortly to be published by Faber & Faber in modern editions.
“They offer an extraordinarily vivid glimpse of a time which will soon not be accessible to living memory. Not only that, they provide evidence of how astutely Mass Observation pre-figured many later intellectual and methodological developments in social research especially in oral history and life history research, in feminist and working class history and in the kind of social research which privileges what we sometimes call the ‘ordinary person’ and the importance of studying everyday life” Professor Dorothy Sheridan, Mass Observation Archive.
I would particularly recommend these wartime finds: