These have been posted on various lists but I don't think they have been
posted here
The National Library of Scotland have added 1st and 2nd Edition 6" OS Map of
England and Wales to their online collection. They already have more recent
1:25000 maps of England and Wales there as well as late 19th Century 60"(?)
coverage of London.
http://maps.nls.uk/os/6inch-england-and-wales/index.html
The NLS also some other series for England and Wales, particularly the
1:25000
http://maps.nls.uk/index.html
http://maps.nls.uk/series/index.html
Map Series of England, Wales and Great Britain:
Ordnance Survey, Six-Inch, England and Wales - 1842-1952 - 37,390 sheets
Ordnance Survey, 1:25,000 maps of Great Britain - 1937-1961 - 2,027
sheets
Ordnance Survey, One-Inch Revised new series, England and Wales,
1892-1908 - 691 sheets
Ordnance Survey, One-Inch New Popular edition, England and Wales,
1945-1947 - 114 sheets
Ordnance Survey, One-Inch Seventh Series, Great Britain, 1952-1961 - 190
sheets
Ordnance Survey, "ten mile" Planning Maps of the United Kingdom,
1944-1960
Not Yorkshire but these are worth looking at
Town plans:
Ordnance Survey Five feet to the mile, London, 1893-1896 - 753 sheets
They have an excellent range of maps of Scotland online including the 25" OS
which I have not seen online in England and Wales so just hope they extend
coverage to include that but you can look up the sheet numbers.
http://maps.nls.uk/geo/records/
There are also some other projects
http://maps.nls.uk/projects/index.html
Martin Briscoe
Fort William
martin(a)mbriscoe.me.uk