Plymouth: A New History
by C. Gill
Lost Plymouth: Hidden Heritage of Three Towns
by Felicity Goodall
Blackshirts in Devon
by Todd Gray
A History of Plymouth
by Llewellynn Jewitt
Secret memoirs of Robert, Count de Paradès containing an account of his successful transactions, as a spy in England, with the real causes of the failure of the ever memorable expedition against Plymouth, in 1779
by Comte de Parade
Bloody British History Plymouth
by Laura Quigley
by Chris Robinson
Plymouth from Old Photographs
by Derek Tait
Aviation in South West Britain 1909-1979
by Dennis Teague
by F W Woodward
Victorian Forts Society
This page has excellent datasheets about all the forts in and around Plymouth.
Brian Moseley has put together an impressive encyclopedia of Plymouth's history.
Bere Ferrers' 1917 Railway Tragedy
This site gives a good overview of the tragic events surrounding the accident that killed 9 New Zealand soldiers in 1917.
This site examines some of the lesser known nooks and crannies of Plymouth's local history. It has many fascinating photos and descriptions of all sorts of unusual places in and around the city environs.