When I lived in Essex one of my regular haunts was the Oxfam bookshop in Saffron Walden.
Yesterday I went back. Browsing through the shelves I found this copy of the Encycolpaedia of Narrow Gauge Railways by Thomas Middlemass. It’s got lots of information about NG lines that I know very little about, for example, English tramways with gauges of around 3′ and Irish narrow gauge lines. This book will be a great way of learning more.
Also, it contains many photos that I’ve never seen before. Look at this lovely image of the Welshpool and Llanfair in the GWR era.