Digital collections such as those listed below include many texts about the history of London.  Most of these collections allow you to search terms in the full text of the sources provided and to see a digital image of the original.

Primary sources in their original form are hard to access from Chicago. If you have the opportunity to travel to Britain:

The National Archives (formerly the Public Record Office) is rich in government materials and official correspondence.  The British Library is terrific for private correspondence.  In either case, their catalogues are searchable online.