Many books are at a substantial discount from

Detailed History of British Railways Standard Steam Locomotives: Tank Engine Classes
Paul J Chancellor, R K Taylor (Editor)
(December 1997) Railway Correspondence & Travel Society. ISBN 0901115770
RCTS books are excellent and full of  information.

British Railways Standard Steam Locomotives
Railway Correspondence and Travel Society
(January, 1994) Hardback. ISBN 0901115819

British Railways' Steam Locomotives -
F.G.Cockman
80 pages 3rd Ed (March 1998). Shire Publications Ltd. Paperback. ISBN 0747803722

The Heyday of Stewarts Lane and Its Locomotives
R.C. Riley
Hardcover - 78 pages (May, 2001)
Ian Allan Publishing; ISBN: 0711027919

Locomotives and Rolling Stock of the London, Tilbury and Southend Railway
~R.W. Rush. Paperback - 138 pages (October, 1994) The Oakwood Press and Oakwood Video Library; ISBN: 0853614660

An Illustrated Review of Midland Locomotives from 1883 Vol 3: Tank Engines
~R.J. Essery, D. Jenkinson
Hardcover - 144 pages (1988)
Wild Swan; ISBN: 0906867665

Steam Locomotives 1955- 40000 - 59999
~Eric Sawford
Hardcover - 187 pages (21 August, 1997)
Sutton Publishing; ISBN: 0750913134

Steam Locomotives 1955: 70000-90774:
Standards and Austerities -Eric Sawford
159 pages (September, 1998). Sutton Publishing
Hardback ISBN 0750916176

Locomotives of D. Earle Marsh
~Charles Fryer
Paperback (June 1994)
Sessions; ISBN: 1850721513

Locomotives of the Ministry of
Defence
~Roger Hateley, Industrial Railway Society
Paperback (1992)
Industrial Railway Society; ISBN: 0901096717

Meyer Articulated Locomotives
~Donald Binns, Gunter Koch
Paperback - 112 pages (June 1997)
Trackside Publications; ISBN: 1900095076

British Locomotives
~C J Bowen Cooke
Paperback (November 1979)
Gresham Books, Henley-on-Thames;
ISBN: 0905418727
An excellent history of the 19th century steam locomotives

British Railways Steam in Retrospect
Eric Oldham
Hardback - 128 pages (April, 1998)
Sutton Publishing; ISBN: 0750917687