The Royal Institute of British Architects.
Online catalogue of books, images, drawings and photographs:
http://www.architecture.com/LibraryDrawingsAndPhotographs/Home.aspx
British Architectural Library Catalogue: http://riba.sirsidynix.net.uk/uhtbin/webcat
Some of my favourite images:
Church of St Nicholas, Burnage, Manchester
http://www.ribapix.com/index.php?a=advanced&s=item&key=XYToxOntzOjM6IjAwNyI7czoxMDoiTWFuY2hlc3RlciI7fQ==&pg=1
Manchester Town Hall: the grand staircase, 1877
http://www.ribapix.com/index.php?a=advanced&s=item&key=XYToxOntzOjM6IjAwNyI7czoxMDoiTWFuY2hlc3RlciI7fQ==&pg=17
Design for an organ case on the choir screen, Manchester Cathedral: east elevation
http://www.ribapix.com/index.php?a=advanced&s=item&key=XYToxOntzOjM6IjAwNyI7czoxMDoiTWFuY2hlc3RlciI7fQ==&pg=129