Ordnance Survey Maps of England and Wales - Revised: Kent illustrates the parish boundaries of Kent when rural districts were still in existence and before Greater London came into being. The map publication year is 1931. An earlier map of 1900 may also be useful. The maps blow up to show all the parishes and many of the small villages and hamlets. Maps in this series are now downloadable for personal use.
Census records for Kent are available on FamilySearch, Ancestry and FindMyPast. The first site is free; the other two are pay sites but have access to microfilmed images. Steve Archer produced a very useful round-up of the available sources, but this information may not be up to date.
Registration Districts in Kent for the period 1837 to the present. By drilling down through the links you can follow any parish through the registration districts to which it was attached.
Volume 2 of the Victoria County History of Kent (published 1926) is available online through the auspices of British History Online. It includes accounts of the early history of Canterbury and Rochester cathedrals, and of several sites now within the conurbation of London.