In addition to the titles mentioned in the bibliography of 'Miniature Railways', the following are of interest:
Butterell, Robin. The Little Giants. Published by Rail Romances. A very limited edition. Superb quality and a good investment.
Jacques (ed.). Romney Remembered. Romney Hythe & Dymchurch Railway Association.
Jenkins and Jenner. The Ratty Album - Volume 1. Ravenglass and Eskdale Railway Preservation Society Ltd., 2002. ISBN 0-9542798-0-8 A superbly produced album of historic pictures taken between the 1890s and the 1930s - with many views covering the "miniature" era from 1915 onwards. Also several new pictures of the Heywood engines.
If you want to know more about books on miniature railways, go to Simon Townsend's pages - the Rhyl Miniature Railway - or Adrian Sant's Bookshop. Links to these are here.
Miniature Railways can be ordered here.