Going Minimum Gauge - a guide to modelling narrow gauge railways of 15" gauge and under in 7mm scale by the 7mm Narrow Gauge Association, is a handbook based on the Associations members-only publications Narrow Lines. I bought myself this 1999 edition from e-bay.
The 28 page handbook is a superb introduction to this genre and although not including much on 18" gauge industrial railways (my main interest) has been a great read. My plan is to use the format/contents as a blueprint for a couple of Blog entries detailing my own OSO Salt Narrow Gauge layout and interest so look out for some new Blog posts soon.
I have in the past been a member of the 7mm Narrow Gauge Association and continue to purchase odd issues of their members magazine Narrow Lines. In fact I have quite a collection of their magazines, many with comments or articles written by me and securely stored in the attic. For more details of how to join, please see this link.