The Victoria County History describes the history of Edgware. Mill Hill, Burnt Oak and the Hale are to be found in the Hendon chapter.
Motco's Cary's 15 Miles Around London (1786), is at one inch to the mile, and gives an interesting overview of Edgware and Mill Hill. For 1870s Victorian Ordnance Survey maps there is www.old-maps.co.uk. These can be searched using postcode and place name searches. There is also www.british-history.ac.uk versions of Ordnance Survey maps which are quicker to open.
The University of Leicester has made available their collection of directories online. There are four Kelly's Directories which cover the Edgware area. For Mill Hill see Hendon. For Burnt Oak, Hale, and Edgware see Kelly's Directories for Middlesex.
There are a number of very good online libraries providing historical pictures of the Edgware and Mill Hill area. Simply use the search facility with words like: Edgware, Mill Hill, Burnt Oak and other relevant search terms.
For 20th century images try Corbis Picture Library and the new 20th Century London website with pictures from the Museum of London, and the London Transport Museum. Francis Frith provided images from the post World War Two period. For films from 1910 to the 1960s try British-Pathe.
Books on Edgware and Mill Hill
Hendon, Childs Hill, Golders Green, and Mill Hill; a Pictorial History, Stewart Gillies (MA) and Pamela Taylor, (Ph.D). Phillimore and Co. Ltd, 1993
A History of Hendon, John Hopkins (Hendon Borough Council) 1964
A History of Hendon, Edward T Evans, Hendon, 1890
Mill Hill, A Thousand Years, Ralph Calder, Mill Hill Historical Society, 1993
Mill Hill As It Was, Clive Smith, Memories, 1973
Mill Hill As It Was, Clive Smith, Memories, 1990
Old Edgware In Camera, Alf Porter, 1991