In 1870-72, John Marius Wilson's Imperial Gazetteer of England and Wales described Assendon like this:
ASSENDON, a liberty in Pirton parish, Oxford; 4 miles NW of Henley-on-Thames. It has a post office under Henley. A land-spring here breaks out after a run of wet weather; and is so copious as to inundate neighbouring low tracts.
Additional information about this locality is available for Bix
A Vision of Britain through Time includes a large library of local statistics for administrative units. For the best overall sense of how the area containing Assendon has changed, please see our redistricted information for the modern district of South Oxfordshire. More detailed statistical data are available under Units and statistics, which includes both administrative units covering Assendon and units named after it.
GB Historical GIS / University of Portsmouth, History of Assendon in South Oxfordshire | Map and description, A Vision of Britain through Time.
Date accessed: 30th January 2015
Click here for more detailed advice on finding places within A Vision of Britain through Time, and maybe some references to other places called "Assendon".